BMW Fxx Series Seat Wiring Harness Diagram + Pin Layout

If you're swapping seats round or retrofitting, then you will want to keep electric movement functions and may even want to retain the heated-seat feature or even safety restraints. Here's a guide to wire colours and the pin-outs on the multi-block plug. 

Pin:

5+6 - CAN data wires to CAS [ Green + Orange/Green]

7 - 12v Power Wire [Red, Red/Blue or Red/Brown, 3mm thickness] 

8 - Main Ground Wire [Brown, 3mm thickness]

Seat Cushion Heating Power+ [Green/Violet or Blue/Violet]

Seat Cushion Heating Ground- [Brown or Brown/Orange, 1.5mm thickness]

Seat Cushion Temp. Sensor [Grey]

Seat Back Heating Power+ [Violet or Green/Violet]

Seat Back Heating Ground- [Brown, 1mm thickness]

F10/F11: Seat Fitment Guide - Differences between 10-13 cars and later models.

In 2013 BMW changed the floor-pan layout of the F10 5 Series to accommodate X-Drive, Hyrbid-Drive and stuff, which in turn makes the fitment of the front seat mounts different for post-facelift cars. I see a number of F10 guys upgrading to M-sports and a few having this problem, offering seats for newer models to swap for the older type, so if you're about to do the same make sure the seats you get are for your corresponding pre or post-facelift F10. [I was upgrading my stock cloth interior to standard SE leather on the other hand, my car being the only example I've found so far with cloth from factory. A 530d as well, very rare.]

This is the same for M-Sport and Comfort/SE seats of all specs.

This is the same for LCI and non-LCI models.

FRONT SEATS:

These are NOT interchangeable between 2010 - 2013 cars and later 2013-2017 facelift cars.

This is the same for F10 and F11 Touring models.

M5 seats of correct model year variant will fit, but features like active corner-restraints are lost.

Front seats from E60 5-Ser and other model BMWs do NOT fit and would require adapting [see bottom].

RUNNERS:
The runners on pre-2013 seats are spaced significantly further apart than those from later cars, meaning a straight swap is impossible. It is possible to adapt the seats / floorpan to accept these seats, as I will discuss at the bottom of this page, but in reality forget it.

WIRING:
The wiring plug fits, but changes to the seats do not work with the safety-system. Active Head-restraints are removed on the post 2013 front seats and although this can be bypassed with the 2-ohm resistor trick to get rid of the error message, there may be another airbag related error coming up and it is unclear whether any of the safety systems in the seat will function properly. The seat-movement electrics all work as normal though.

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If you don't know the model year of the seats you're buying then here is what to look for:

Control-levers - Pre-2013 seats have the classic handles built into the seat-surround trim with the white logos, whereas post-2013 seats have plain black handles sticking out from under the centre of the squab.

Head-rests - Pre-2013 seats have squarish split head-rests with a button on the side to extend half of it forward, whereas post-2013 seats have slimmer, one-piece head-rests with visible stitching on the front panel.

M-Sport - These are identifiable in the same way. However, the control-levers are similar to those from the E60 M-Sport seats, which will not fit. E60 seats have brackets on the runners that stick outwards and the head-rests are plain/round, as pictured, so make sure you don't end up buying a cheap set of those instead.

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REAR SEATS:

Straight swap. These are totally interchangeable on all F10 models from 2010-17.

F10 rear seats will NOT fit the F11 Touring, however, as they have the split/folding back. [The shape of the seat is the same though, so it would not take much to adapt them to fit and give the F10 split/folding rear seats! This is something I would really love to try out in future.]


DOOR CARDS / TRIM:

The door cards are interchangeable between all F10 and F11 Touring models. Curtain-airbags are now in the seats, not the doors, so swapping the wire over is easy and does not differentiate between model specs, LCI etc. [NOTE: With F11 Touring, the rear door-cards do not have grilles for the door-speakers, so they will need swapping over from your original ones. This is something else I would love to try out - putting rear door speakers in there F10!]

Most of the dash and centre-console trim is a straight swap also. The only change to the post-2013 cars being the centre-console switch-panel - older ones having the ash-tray at the front and a poorly designed cup-holder in the centre by the E-brake. Later ones have a double cup-holder at the front and a smaller ash-tray / cubby-hole by the E-brake. These panels are interchangeable, as the console itself is the same, though I'm not sure why you would want to.

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FRONT SEATS RETROFIT:

Adapting the floorpan or seats to fit the post-2013 seats is a possibility, though the level of effort required makes it a bit pointless. The first thing to consider is safety - will they stand up to a crash or even pass an MOT? The other things are the seat not quite being in the right position and the areas where the carpet doesn't match up will look ugly. If you're sick of waiting for the right seats to come along and the above things don't deter you then there are some possibilities, but I have not tried any of it out or would recommend it...

The bolt-hole spacing on the pre-13 seats is about 100mm wider left-to-right, but the spacing front-to-rear is the same on all models. This doesn't mean you can use the mounting points on one side and just adapt the other side to fit, as works with other BMWs, since the pre-13 bolt-holes are just too close to the sill / trans. tunnel. The difference in width is not central to the seat either [as in ~50mm equal either side], so adapting to fit is not going to be easy.

Adapting the Seats:
Making a one-piece adapter from thick sheet-steel would be best, similar to the ones that Sparco etc. make for other cars, that bolts to the floorpan and has more holes further inboard to bolt the seat to. These would be difficult to make without proper equipment, though an engineering firm will probably knock two up for cheap and with some black paint they would look OK.
Making one out of slim strips of box-section steel would also work, welded into a square or even just a left-to-right strip across the bolts front and rear. Either way, all the above methods will raise the seats slightly higher than usual, though lowering the seat fully might account for this.

Modifying the Floorpan:
New holes can be drilled at the correct positions in the floorpan and new nuts welded in their place to accept the seats. By the look of it, the floorpan has just enough room to accommodate this, but if not, the raised sections of floorpan with the captive-nuts for the seats could be cut out and welded back into place slightly further inboard. This sounds hairy, but would not actually be a huge undertaking, though you would seriously have to know what you were doing to get it right! 

F10: Rear Parcel-Shelf Swap / Replacement

The original parcel-shelf was in a sorry state, with several rectangular slots cut in it where Police lights had been fitted. The two in the below picture were glaring at me every time I looked in the rear-view mirror and third, twice the size and facing the rear, could be seen clearly from outside. Despite not being one of the first things to get replaced, it became my biggest concern with the interior and has ended up being the piece that has made the most difference to the car's appearance.

A mint and very clean  replacement was £60 from eBay and came with the speakers, grilles and all trim pieces, handy as mine was missing some.

FITMENT:

1. Remove the rear-seat squab by pulling the front edge up sharply at either side and sliding it forward.

2. Remove the 3 bolts holding the bottom-end of the rear seat-belts to the body using a T45 Torx-socket.

3. Remove the rear-seat back-rest by undoing the 2 bolts holding the bottom brackets in place using a T45 Torx-socket and lifting it vertically up off the upper brackets.

4. Remove the 5 plastic-clips holding the leading-edge of the parcel-shelf by prising the centre-pin out with a flat-screwdriver [or ideally a trim-tool] and then prying the clip out.

5. Loosen both of the C-pillar trims. They are only clipped on and should pop off easily enough at the bottom. They do not need to be completely removed, just loose enough to allow the lip at either side of the parcel-shelf to slide in and out.

6. Slide the parcel-shelf forward partially, pop the seat-belts through the apertures and disconnect both speaker wires. The parcel-shelf can now be fully removed.

7. Swap trims/speakers over to new parcel-shelf.

8. Reverse steps 1 - 6 to refit.

F10 530d: Correct front seats acquired!

Managed to find the right pre-2013 front seats on eBay to replace the incorrect fit ones [https://beemerlab.blogspot.com/2017/05/f10-530d-black-leather-rear-seats-and.html]. They were for sale on their own, the owner having swapped his front seats for M-sports and was willing to let the old ones go for just £140. They were from a 2011 car with 80k miles and in very good nick, only snag was they were in Northampton, so I removed the passenger / rear seats and did the 6 hour round-trip to collect them. They bolt straight in and bring up no air-bag warnings, so that's problem solved  all round.

The post-2013 front seats I was selling on eBay had attracted quite a bit of attention and a few offers, but eventually someone paid the asking price of £180. This was mainly to convince me to leave them with someone to meet a private-courier organised by the buyer, which was a bit of an inconvenience, but they sold regardless. This leaves me £40 in profit after all thanks to the error, save the fuel money travelling to Northampton.

E21: Seat Fitment Guide:

Swapping seats round in a BMW E21 3-series is a nightmare, thanks to the floor-mounts being two different heights. To fit level-based seats from another car requires chopping out the welded floor-mounts and once this is done there is no going back.

1. Check if your floor-mounts have been chopped:

Check the outer floor-mounts on either side, spot-welded in place just inside the door-sill. They should look the one in the left photo below.

If they don't, and they look more like the right photo [with or without the wood], then you have chopped mounts and seats not originally from an E21. You also won't be going back to E21 seats very easily, so to preserve the style and make the ride a lot better, throw in seats from a newer BMW.

2.1 Fitting stock BMW seats from E30, E36 etc. :

This makes sense. E21 seats are sprung and uncomfortable and were dropped in almost every succeeding model. Other BMW seats are easier to come by and cheaper. E30 seats preserve the look almost identically and are far superior in construct. They also fit the inner seat mounts on the E21, the ones against the trans.-tunnel, so only the only outer ones need adapting. I am sure this is the case with seats from other 80s BMWs, possibly the E36 and E46 as I've seen it done, but need confirmation.
Modifying the outer floor-mounts, the ones against the door sills, is impossible as they just sit that much higher than the inner ones, the E21 of course having a crazy odd-height sub-frame design, so they need cutting out. Putting in new ones is pretty straight forward though, you can fabricate them with steel or alloy C-channel, wooden blocks, improvised stilts - these are all popular methods. Tilt-forward seats don't tend to be very strong anyway, so the new mounts don't have to be rock solid as long as they're bolted or welded securely to the floor-pan. Mine used the stock inner floor-mounts and a wooden block on the outer side to get the height - this presented no problem to the UK MOT (roadworthiness-test).

Check out my guide here: http://e21dub.blogspot.co.uk/2013/09/e30-sports-seats-fitted.html

Or check out HSVTurbo's guide HERE [where I got my idea] in THIS THREAD on bimmerforums.

2.2 Fitting rigid Racing bucket-seats:

If you're building a track car then it's obviously best to keep things as stiff and secure as possible. The neatest and most weight-saving method is to cut some steel cross-beams to fit and weld them across the floor-pan, from the trans.-tunnel to the door-sill. This could be done with or without removing the outer floor-mount, but you will probably have to do this to get the seat low enough. Two cross-beams under each seat would probably be best, but it is most often done with just reasonably thick one.

More info. HERE on bimmerforums.

2.3 Fitting non-BMW stock seats (VW, Ford Recaro etc.) :

To get the right height and a central-position with stock seats from non-BMWs there is a lot of modification to the mounts - these seats will always have the bolt-holes in the wrong place. The tidiest way would probably be to weld in rigid cross-beams like in the above guide [2.2], otherwise you will need to fabricate custom mounts. [I used a combination of wood-block and steel cross-beam to fit Ford RS Recaros to mine in THIS POST].

Check out my guide here: http://e21dub.blogspot.co.uk/2013/08/recaro-seat-base-adapter-finally-solved.html

3. Re-fitting stock E21 seats:

I have never seen anyone online do this. If you have level floor-mounts then I see no reason to switch back to E21 seats. All but the Recaros are uncomfortable, sprung nightmares. They give a truly bouncy and uncertain ride - only the most restoration mad would want it. The Recaros are super-rare and overpriced to fit the E21s mounts. E30 Recaros are a fraction of the price, have level mounts and are better anyway. If you are so desperate to restore your E21 dutifully, then I would consider it having cut floor-mounts a deal-breaker and to find one with stock seats, because the car will never be the same again.

There are 3 ways I see to achieve refitting stock seats:

1. Build custom outer mounts using a combination of the methods above, building them back up to the original and rather tall height for the stock seat-frames. [].

2. Locate a pair of original outer floor-mounts that have been cut out of another E21 and refit them to yours. These would first need to have been cut out carefully from the donor car and preserved by someone. Whether they would advertise them for sale is another matter, but asking on the forums can't hurt. Otherwise, you'll need to find someone with a scrap / breaking E21 and ask to neatly cut out its floor-mounts for a few quid. Either way, obtaining a useable pair is going to be tough.
Once you do though, they would just need to be measured up to where yours were cut off and welded into place. How neat a job you do will determine whether they fit or if it's unnoticeable.

3. Fabricate new outer mounts from the ground up out of steel and weld those in. I will endeavour to make a note of the dimensions of my cut one before passing it on and do a quick CAD design to fab-up a simple one.

**Also, I still have the passenger-side (N/S, Euro/US Driver-side) floor-mount [above], so if anyone wants it to try and weld back in message me! [Sadly, I cut the driver-side one out more severely and this is probably going to be the case if yours are chopped as it gives the wood or whatever more to grab on to.]

E21 316: Update + MOT Pass

After my disastrous few weeks I managed to get hold of a Bilstein strut-insert and get it onto the car along with the longer springs, meaning it was finally back on the road and in an state worthy of the MOT test. I cautiously rolled down to the test-station on Saturday and am glad to say the E21 passed with flying colours!

Bilstein damper is strong, but will be a pain with the short springs.

This was a relief considering the slightly dodgy seat-frames I made bolted through the floor-pan, which stuck out like a sore thumb when it was up on the ramps, along with the scuff marks where I've grounded the underside, but with the car at a more standard height off the ground this didn't see to bother them and there was no problem with the chassis at all.

Oddly, the guy told me that the split-pins were missing from the castle-nuts on my steering track-rod ends and this would actually be a fail under normal circumstances, but they could see I'd been working on it and decided to give me the benefit of the doubt. This was true, I'd had them off swapping the springs and hadn't bothered to replace the split-pins as they would be coming straight off again to put the short springs back. That was decent of them to let it go, as it would have been beyond frustrating to lose another week to something so trivial and shows just what a pain the UK MOT test is.

Folded up and with a 1/4 inch thick layer of
pallet-wrap to send as a parcel.

Spending on the E21 has been a bit out of control since buying the E30 seats and it is still in desperate need of the wheel-arches finishing off. By this point, I'm past what I can recoup if I sell the car. I'm still using my E39 as my daily, the E21 has had no use for nearly a month with no MOT and finally, due to a planned move, it's looking like I will have to let her go anyway, unless I can find somewhere cheap enough to store it. I had hoped to have the arches skimmed off and painted by the guy who did the respray, but money is tight so I will be buying a rattle-can and doing the best I can with them.

Thankfully, I've managed to generate a bit of extra cash to pay for the strut-debacle [this post], by selling some bits on eBay, like a battered old set of alloys, an E36 nose-cone and other bits I had lying around, as well as the original beige passenger seat from the E21 - I think I have enough seats for the car now. I hope the seat arrives to the buyer OK, as I had to practically shrink-wrap it to be able to send it Parcelforce [£27], pictured, as the original method of courier was just over £60!

My spare set of E21 ash-trays is not selling on the other hand and is still on the site here - http://www.ebay.co.uk/itm/181235987123.

E21 316: Fitting E30 rear sport-seats - or having a go anyway.

Stock fit works, but leaves big gaps.

As with the front seats I used HSVTurbos build [here] as a rough guide. His rear bench-seat is the all-sponge kind, whereas mine have the sponge base, but a sprung back-rest, possibly due to the folding arm-rest or model year difference. This means I have a metal frame to work with on the back-rest, so it can be pressed flat to greatly reduce the proud fit at either corner, but also means more needs to be cut away than just blobs of sponge. The E30 coupe's rear seat-pan, the body-tub and front doors are almost identical dimensions to the E21, so the rear seat-base just slots in. There are no brackets along the bottom-edge to clamp the squab down, but with it seated flat and wedged under the back-rest these may not be necessary, although fabbing some up might be worthwhile to stop the carpet sagging down in the footwells.

The general shape of the back-rest is more or less the same as the E21 and the hooks fit into the existing brackets, though the E30's parcel-shelf is curvier, causing the back-rest to stick out slightly at the top corners. The back-rest also sits slightly higher, leaving an inch gap at the bottom between it and the squab. If these gaps don't bother you then the seat works fine like this and will only need a little chopping off each upper corner to achieve a pretty good looking fit. If they do, then a little more work is required to seat the back-rest flush and minimise the corner gaps.

The main problem here are the seat-belt reels, which protrude from the bulkhead and fill up where the upper corners of the back-rest want to be. The back-rest bolsters are also slightly too broad, though the centre of the rear frame is more or less a perfect fit to the E21 bulkhead.

Modifying the Back-rest:

1. Bend up the small hooks holding the cover material in place over each side edge of the back-rest and pull the fabric clear of the metal frame.

2. Cut away the side-support on each side of the frame, leaving just the edge of the 'mattress' frame that holds the springs, as in the photo below. The brackets at each bottom corner should come away here too.

3. Cut in half the metal bar that runs between the centre and bolsters, the one attached to the top spring in each corner, and trim away the top spring as close to the frame as possible.

4. Cut the metal bar running along the top of the frame about 4" in from either top corner

5. Cut away the boar-hair matting along each side of the back-rest to reduce its depth by about half.

6. Cut a 2" square from each top corner of the boar-hair matting to give a recess where the seat-belt can run freely back into its reel.

7. Replace the fabric cover, hooking it to points on the mattress-frame and tucking it under the springs. At each top corner, I pierced a hole and ran a cable-tie through to hold the folded fabric clear of the seat-belt reel.

Fitting to the E21:

To get the back-rest sitting flush to the seat-base the original mounting-brackets may have to be abandoned in favour of slotting the hooks on the bulkhead directly into the top of the back-rest frame. The original lower brackets are attached to the frame here and may need cutting off for a tight fit.

1. Hook the top lip of the back-rest frame over the metal hooks on the bulkhead. Press hard to make sure the metal bar gets over the hooks and not just the boar-hair and fabric. Give each side a good knock down onto the hooks. The top of the back-rest should be about 5mm lower than the parcel shelf.

2. Fit the centre arm-rest and lock the clips into place. The back-rest should sit a bit proud of the bulkhead and need pushing flat. Drill two holes through the back plate of the arm-rest and bulkhead. Fasten it together with two bolts or pop-rivets. Not only will this hold the seat in place, it will also stop it moving when the arm-rest is lowered.

E21 316: E30 Sports Seats vs. Re-covering the Recaros

Saw these absolutely divine Recaro sports-seats from an early E30 Baur cabriolet on eBay. They're all but identical to the Recaro/sports-seats from the E21, but have different subframes for the E30, not that it would make any difference now I've started cutting out the stock mounts and the inner side of the E30 base fits perfectly to the inner-mount anyway, so would be a bit neater/stronger than my all-custom base. Only snag is, the price is a little more than I'm willing to pay as the rear seat and door-cards are included. I've got them down a bit, but I'm sticking to my upper limit and they're having none of it. 

It got me thinking, the shape isn't too different from the RS Turbo Recaros I have, it's just the colourful Ford fabric that doesn't suit, but there may be a way to create some ultra-cheap 'replicas'. I figure the stock E21 seats aren't worth much at all, so there may be enough decent fabric left on the two to re-cover the centre-sections of the Recaros in beige cloth. This leaves the bolsters/head-rest grey alcantara, which is where fabric spray-paint comes in, about a tenner a can off eBay, to make them dark-brown. I whipped the cover off one of my E21 seat to weigh it up and it does look possible. On the other hand it has the potential to mess up a great pair of Recaros that are worth quite a lot more still than I previously thought, as well as condemn a tatty but useable pair of E21 comfort-seats, which I could at the very least donate to someone. At the risk of the end product looking naff, I don't think it's worth the sacrifice.

** UPDATE - Another pair came up on eBay yesterday and guess what, the guy emailed back about the original set accepting my offer. The second pair are a bit dirtier and the back seat is wrecked, but the seller will come to about half the other offer, only thing is he wants collection and they're in Cardiff. It will cost me the difference to drive down there, plus a day spent sat in the car. To be worthwhile I'd have to get this second set for buttons, so I called up the original guy and bought the set above - they'll come Monday and I can use the rear bench. I'm glad this seat problem is finally put to bed as the MOT is a week on Saturday and I need perma-fixed seats!

E21 316: Recaro Seat-Base Adapter - Finally solved!

I'm all for keeping things so they can be converted back to stock later on, but fitting seats with the E21 base-mounts is a nightmare. Once you've overcome the different height on each side and the mounts being too far apart, the seat has to be set low enough to be able to drive the car. I've tried a couple of times and just can't deem a complex two-height setup safe enough to be permanent. The Planted adapters from this post are just too expensive - I don't have £300 to spend on brackets for seats I had lying around so, with regret, the outer [higher] mount has had to be cut out. Gleefully this has done the trick and the passenger seat is now mounted at the perfect height and slides back and forth like it's on ice, so with a nod to retaining originality, this is the first thing anyone wanting to fit sportier seats should do.

Ripping out both mounts gives a blank canvas to put any seat-frame in any position, but a bit more research taught me that the inner mounts by the trans. tunnel can be re-used when fitting upgraded BMW seats, - Recaros, E30 seats etc. - which is what I would fit in future ideally, so they stayed and the outer mount came out to reveal 5" strip of flat surface, the only bit in this area of the floor-pan. Given the choice I would use steel box-section here to raise the flat section up to the same height as the inner mount and do some welding, but I just cant find it anywhere locally in the right size. I got some immense inspiration off this thread on Bimmer-Forums - http://forums.bimmerforums.com/forum/showthread.php?1757170-E30-sport-seats-in-e21 - in particular this guy, who used a block of wood - http://forums.bimmerforums.com/forum/showthread.php?1584670-HSVTurbo-quot-Mac-quot-Build-Thread. Would had been considered originally with the Recaro project, but not ruled solid enough. HSVturbo showed it can work here though and I guess as long as the bracket is bolted well through the floor, the would should be plenty solid as a riser and won't twist about.

It transpired there was a 2x4 type beam leant up in the shed that was pretty much the exact height of the inner seat mount, so I chopped off a 55cm piece of it, to match the full length of the inner mount - I can see the extra PVC-pipe thing works for HSVturbo, but one piece has got to be more secure. I then ran a length of angle-steel from left to right at each end of the inner-mount and bolted it through the existing holes. The angle-steel also sits flush against the front and rear face of the mount and wood-block. Lastly, the angle-steel is bolted through the wood to the floor-pan. Now I have two cross-bars to bolt practically any seat onto, good enough to rival the Planted adapter, well not quite, but it's an inch lower, solid and cost a whopping £6.99 to make. The carpet even fits snugly back over it, with a few slits for the cross-bars to go through.

To mount the Recaro on the cross-bars the base-mounts needed removing entirely. These are held to the sliders some of the strongest rivets I've seen and needed grinding and chiselling out, but thankfully not enough to mess up the holes, which I re-used to mount it with short M4 bolts. For the seatbelt-buckle anchor I just used a short piece of angle-steel, but it will need replacing soon with either a right-angle bracket at least 2.5mm thick, or better still some stalk type buckles from an E30. Some E30 Recaros would go nice with those too...

E21: Custom Seat-Base vs. Planted Adapter

Had a go at fitting the passenger Recaro seat, this time using the Ford sub-frame and slider instead of the BMW one. The driver's seat I did last year has sunk into the BMW slider and won't adjust anymore, not to mention the seat is too high. OK, I've got used to it and it's holding for now, but bolting things on top of the E21 slider/base plain doesn't work, is ugly and will need improving before the MOT. Using box-section etc. that won't sag down is out too - my head would be pressed against the roof.

For this side I chopped the left side of the Ford subframe so it would fit on the floor against the original outer seat-mount. This mount is an over an inch higher than the inner mount next to the trans. tunnel and this just adds to the problem, as the mounts are too wide apart for most seats anyway, so simply raising one side is no good, you need cross-bars. Cutting the Ford sub-frame and bolting it through the floor-pan on the outer side is the only way I could see keeping this mount, so stock seats could be later refitted, and without it all being set too high. The inner mount is just the right height, so I reused the bolt-holes to mount a piece of angle-steel, which the seat then bolts on to about half an inch further out. To bolt on to the angle-steel, the base-mounts had to be ground and chiselled off to leave just the sliders on the right side, which was a right pain. The placement of the seat was now perfect, but the funny shape of the E21s belly-pan, bespoke Ford mounts and custom-made risers did not make for the strongest design. The seat felt solid to sit in, but once I'd moved it back and forth a few times I felt the left side mounts twist and the sliders jammed. You could now feel it moving around a bit, so without a safety pass this is another fail.

It seems that there is just no way to keep the stock mounts and be seated nicely, unless you have better tools and materials than us and can fabricate a very flat, snug fitting adapter plate, with feet on one side and a seatbelt bracket. Believe it or not, they exist, made by a company called Planted in the US, available from a few sites over there [http://www.stableenergies.com/Planted/products/393/] and eBay [http://www.ebay.co.uk/itm/Passenger-Seat-Bracket-for-MOMO-NRG-SPARCO-RECARO-BRIDE-BMW-E21-Series-1975-1979-/120994172571?pt=Motors_Car_Truck_Parts_Accessories&hash=item1c2bd08e9b], though pricey at £150, each!

They're very nicely finished in 3/16" thick steel, bolt straight into the existing mounts and have a series of oblong holes to drop most types of sport seat onto, Sparco, Bride, MOMO, NRG, Recaro are menioned, though you don't get sliders. I've seen one with sliders made by Corbeau [http://www.harrisonmotorsports.com/corbeau-seat-brackets.html] available through Demon Tweeks, again £150 and will likely need adapting to fit non-Corbeau seats. I've been weighing up buying these for the best part of a year, so to weigh up the pros and cons they're an easy straight fit and let you keep the stock base-mounts, but with sliders on top the seat still sits too high and the crazy £300 cost for the pair is getting into E21 Recaros territory, so really I think it's worth having one last go at a custom adapter.

E21 316: Recaro seat fitting, 1st attempt...