you could always fit a usb power supply and connect it to the interior light supply. Something like this:
http://www.ebay.co.uk/itm/iPhone-Ba...30?pt=UK_Motorcycle_Parts&hash=item27ea0a237a
http://www.ebay.co.uk/itm/iPhone-Ba...30?pt=UK_Motorcycle_Parts&hash=item27ea0a237a