I use a Powerlet lead, taking power from the front accessory socket (Adv), into the tank bag and then to the iPod via a cigar lighter type car charger.
You should have no prob with 12V versus iPod. There's lot of car chargers for iPod, just get the minijack (that's on your BMW) to standard carsocket adapter (i.e. i got one from Touratech for around 10€) and you're done.