Ive never seen anyone do an aftermarket cd player install that was able to go under the wood trim.
they said there is just not enough room back there. The wood trim has to go.

You can get a harness for the steering wheel controls. I know they had this in my previous car (Maxima).

Who makes the cd player in the BMW? I believe it is Alpine. If it is Alpine then it is probably already top-notch.