The radio module on an X5 with navigation is located down in the spare tire compartment, behind the battery.
Here's the bad news. The radio module in a 2001 X5 doesn't support AUX. I replaced my radio module with one from a 2003 X5. To do so you also need to have a conversion harness, as the 2001 radio module uses round pin connectors and the newer radio module uses a flat pin connector. See
http://www.xoutpost.com/articles/x5/s...formation.html