Hmmm, can "o" worms! Try this site for starters, http://www.bmw-540i.com/1995-540i-cd-changer/index.php. There was another site that was helpful for me, but can't find it at the moment. Also, do a search in the forum for numerous threads and posts about this topic. In my case I had a Pioneer headunit in my '90 535, pre-installed BMW CD cable and found the right BMW cdchanger that worked. The Pioneer cdx-m30 changer (non BMW unit) also worked with my car, but required the cdx-m30 cable instead of the factory cable. My car is a 1990 535. I believe 91's on that also have the Pioneer headunit would work with that cdx-m30 changer, but needed a special adapter. This is a very mixed up topic and takes some clear thinking and research to get the right stuff.
Good luck.
JonE