Alpine makes a CD changer that will work as long as you have the right adaptors. See
http://www.xoutpost.com/showpost.php?p=21708&postcount=1 for a listing of what you'll need.
Two questions I would have are (1) why not just get the newest BMW CD changer that now supports MP3 files or (2) why get a CD changer at all - I dumped my CD changer in favor of an iPod with the Dension IceLink Plus interface.