Was it a stock radio? Then unless you are going to put a stock radio back in you will need to run all new speaker wires. BMW uses some weird common ground type system that sends a signal to the amp in the trunk and then to the speakers. The speakers could probably be reused if you cut the old wires to them and keep the crossovers but you can put new speakers in fairly easily if you're handy. So really all you need to do is find the hot wire for the memory, the switched wire from the ignition, ground and the wire to the antenna power. I don't know what these are off hand but they shouldn't be hard to find.
Good luck.
Steve