03 BMW 325 Radio

If I remember correctly pulling the factory radio out is no biggie, carefully pry the wood panel off from around the top & sides of the radio, then 2 torx screws on the sides of the radio.  Metra dash kit 99-9300

Does it have the upgraded sound system?  If not, then you can use metra's 70-9003 to wire behind the radio (you may be able to use that one even on the upgraded system)  Use a euro antenna adaptor and you're all set!  Just be prepared for some fun times fitting all those wires behind the radio, it's pretty tight back there!