I spoke to my local car-electrics / stereo / phone place about this a while ago - although I can't recall the exact kit he was suggesting, I do recall that there was a wiring harness and gizmo available, and also a phone cradle which could be hard-wired to the car. This would then give me power to the iPh when it was plugged into the cradle, as well as allowing it to interface with the car stereo for music etc. More importantly, it also muted the stereo to make or receive calls. Iirc, he wanted about £250 for the whole job.