If you have an e46, its a piece of cake. BMW made a factory Nav for our cars with CD.

FOr $800 this is a 5 minute swap to get you a CD player:
BMW CD NAVIGATION MONITOR SCREEN E46 323 328 325 330 M3:eBay Motors (item 390044294200 end time Jun-13-09 15:14:33 PDT)


BTW... My CD / Nav unit is from a 2006 and it doesn't play MP3s... Yet my business CD from 2006 does in my other car... so apparently the Nav didn't get the MP3 capable upgrade... Not sure if anyone can confirm this...

looks like it comes with casette, CD and mini disk
RealOEM.com BMW E46 M3 on board monitor / TV amplifier