I have the stock head unit, and I'm wanting to add a cd player and possible a sub-woofer. I've just started looking into it, and just for a laugh I decided to check out the amp... I pulled out the left (looking at the trunk) side carpet, and there was nothing there, I even checked the other side and there was just the jack etc. There looked to be what could have mounted an amp, but no amp and no wiring.

The stock head unit I have has a pyramid-style seek/tune knob, the bmw logo stamped on the tape slot, and the strange slot-type sensor you stick your finger in and move left to right to adjust bass/treb/bal/fade. It has speakers in the rear deck, front kick-panels and dash.

I'm hoping to get a cd changer from a wrecked car, and just add it to the head unit.