When I first purchased my '95 540i, I wanted to save cash and did a complete wiring diagram for the f*cked up stereo system these have in them, so I could just replace a few components. It worked for awhile. To get the best sound, I ended up removing the entire interior and all factory components, and replacing them all with Boston Acoustics speakers, ADS amps, and Alpine receiver and equalizer... along with all new wiring of coarse.

The factory systems in these are wired with all positive and negative speaker wiring broken out into sub, mid, and tweeter. So you have an average of four wires running to each speaker (not including the phone wiring)! I would suggest putting a passive head unit in, or you may damage the factory amp.