The standard BMW Business stereo is a masterpiece and takes up half the dash, but sadly it lacks the RCA-output and remote-signal that are essential when fitting an amp. A fascia-converter is about £10 and a wiring-adaptor about £7, so it's not expensive to throw in any after-market head-unit, but if I do that then I lose my CD-changer, tape-deck and the excellent steering-wheel remote. The cheapest remote-adaptor for Pioneer I can find is £85! and there's no way of knowing if it'll fit my dinosaur model. Worst of all the new fascia would ruin the teutonic look of the dash.
I started checking the forums and found a few people who managed to splice an RCA-cable from the rear-speaker wires, so I thought I'd give it a go. It turned out to be one hell of an involved process. The back seats / parcel shelf have to come out, the speaker-wiring is perplexing and I eventually found a way to get a remote 'on' signal for the amp from a parking-sensor relay. The remote-input on the amp will take any signal up to 12v, so you could just wire it from the battery and have a manual off switch, but at least the fiddlier way means it goes off with the ignition - no flat batteries in the morning. The battery is in the boot too, so there's no need to touch anything up front or run cables under carpets.
The system works brilliantly, there's a LOT of bass there, which I'm quite surprised about with the RCA audio-signal coming off from the rear-speakers, but they are still working fine too. As you can see above, I've also set the subs facing inwards this time so I can make use of the magnificent boot - I'm sure the bass is being stifled a bit this way, the back seats are vibrating like crazy, but it's plenty loud enough either way. The only downside is no separate controls for the subs, so they pump out at the master volume, can't be turned off and receive a completely un-filtered audio-signal - there's some nasty higher frequencies creeping in. My 1000w Alpine V12 amp is a solid mono-block and has no controls either, which is a disaster really because it's the matching unit to my 12" Alpine R subs, but it'll have to make way for a lesser 1k watt Toxic Audio one soon that came with my old Subaru. It's not a pretty amp, but it's got a volume-knob, lo-pass filter to single out the bass and 2 channels, so I can give each sub its own terminal instead of twisting the two wires together. Everything I need basically, but I'll miss that blue-glow screen...
This is where I'd normally put the 'How-To Guide', but the process is too intricate to throw on the wall, so I've given it a page of its own, split into five sections, linked below:
