Detailed step by step instructions are here http://www.bimmernav.com/BMW_Sirius_..._Retrofit.html

Step 1) Replace your battery with a New Gen version from 2003 and up. If you have nav, the radio is under the spare tire. If you do not Radio can be swapped out. The cool thing is you get Aux with the new trunk radio and IF you get a REALLY new dash CD radio (no nav) you might get a nice MP3 player suprise, and support for Aux

Step 2) Wire up Sirius like you would a 2004 X5. It is pretty straight forward and well documented here

Martin