Would be good Idea to try the E60 forums instead of here.
Otherwise, I'm sure you can buy a mp3/audio unit with an aux in on amazon for like $100 and swap it out.
Something like this, though it may look ugly since it might not be exact shape of contour in E60:
http://www.amazon.com/Dual-XD6150-Re...ype=automotive
Otherwise, do some research sure someone made one specifically for the E60, though it may cost more.