Is this easy to do? The stereo is an odd size but I really want to upgrade mine to an mp3 player, have a kenwood one sitting about doing nothing. Is it just a case of buying adaptor so it fits?
its a 98 540i btw.

Yeah there'll be a plastic adapter piece to fit it. Just go to a BB/CC with an install bay and ask them for one. Probably be $30-$50. Is your stereo a double DIN? Meaning is it twice as tall as your Kenwood player? VW was also doing this to be a pain in the ass.

Don't forget you'll probably need a conversion plug for your European antenna plug to a North American plug. Also you may need a plug for the wiring from your speakers to go into the back of the pigtail for the deck.

I know I did with my 92 535I.