I would suggest going to the following page, and then scrolling down to
the stereo section.
I was going to say, and the article I read here would seem to confirm,
that most of the audio problems with these systems can be vastly
improved simply by putting in better speakers.
I have a '98 E36 328is, and the previous owner had an aftermarket
system in it, and pulled it before trading it or whatever. The place I
bought it from had put the original head back in, but the original amp
was missing, and had been bypassed anyway, so there was no sound. I
managed to find all the info I needed to install a new Alpine head unit
and new speakers using the original wiring that was still in the car.
(Thanks to the folks that post this kind of info on the net.) Best of