If your 2001 is equiped with a cassette player a very valid, simple and cheap solution is to use a cassette adaptor. You have to look for the right cassette adaptor though since some have noisy wheels. I have been using a cassette adaptor to connect my mp3 player for several months now, listening to music everyday, and the sound is great. In my opinion this solution provides a better sound than an FM transmitter based solution, comparable to sound coming from my CD changer, assuming of course that the quality of the mp3 files is good. I did try FM transmitters and was not happy with the result.
I passed the cassette adaptor's wire behind the radio unit, inside the central column and central harmrest so as there is no visible wire. My MP3 player is in the harmrest box. Here is the related thread.
http://www.xoutpost.com/x5-e53-forum/...ssette+adaptor