I installed the aux kit to come in right where the seat warmer button is (03 330ci). But like it was mentioned already, you need a bmw stereo head unit that was built after 09/02. The sound quality is stellar and a million people wouldn't know the difference if I was playing a CD or running an mp3 player (I use my ipod) through an AUX cable; no distortion, no buzz, nothing weird....better than paying 40 bucks than shelling out 100 for the dice unit (which I think is a pain to install anyway). Plus, if you're going on a roadtrip w/ a friend that doesn't have an ipod, it's possible to listen to their stuff too.
Good luck on trying to find a compatible mp3/ipod/aux unit!