BMW: Aftermarket Car Stereo, rear speakers, stereo wiring

Question
Josh,

I would like to replace the car stereo on my 91 318is (E30) I seem to recall from my 87 535i (E28) that the grounding system is different. In short is there an adapter that will allow me to plug a modern stereo into the wiring or do I have to rewire the whole system?

Thanks,

Seth

Answer
Seth,
 The stereo wiring on these late-80's BMW's is different from most cars that I have seen.  On the back of most aftermarket head-units there are two wires running to each speaker.  Most head-units supply wires for 4 speakers, so you would have 8 wires.  The BMW head unit only has 4 wires... 2 for each side of the car.  The rear speakers are wired to the front speakers, and the front speakers get their signal from the head-unit.  So in order to take full advantage of a new head unit you will need to run your own wiring.  You would need to run two wires to each of the rear speakers and you could use the existing wires for the front.  However you will need to disconnect the wires running from the front speakers to the rear.  If you do some searching on the internet you could possibly find a harness that will plug into the existing speakers, however I dont think you'll have much luck as the stereo and the car will use different wiring.

 Good luck,
 Josh