You can get a dice electronics "ipod out" adapter for certain vehicles including BMW. Depends on your car. A general google search should help. A few companies make a similar solution, Dice is just one I had experience with in my Acura TL.
In my new car (a Mini Cooper) I decided to replace the stock head unit with a Pioneer aftermarker head that had ipod out already built into it. It also has bluetooth built in so I can make handfree calls in the car through my speakers and a separate mic.