I'll tell you that the rectangular white connector is power input and audio signal output (to speakers or external amp). The small round connector is for the CD changer connection - data in and out and audio signal in. I couldn't find a diagram to help you on this one. If you had the harness from the car, you'd have more information - the wire colors - which I might be able to get you some info on.
I'd try your question on a radio install forum, or maybe a BMW forum.