It does not have an RCA or true aux input but it should have CD changer input. I know people have patched in an ipod in lieu of the CD changer option so I think it can be done. You'll find a lot of info in bimmerforums.com in the ICE forum. It tends to get a little technical for me, hope you're good with a soldering iron.
A decent option are the Nakamichi head units which have RCA inputs and look stock as well as sound amazing. I had a CD400 and loved it. The sound was fantastic even with stock speakers. The downside is their reliability is spotty and they are expensive for what you get.