When the car is close enough to home to access the wireless network, the RPi can sync a music folder or collection with what's in the car.
Having a USB stick or small drive or iPod situated in the car to hold the music and either use the iPod to control (via http etc) the music output from the RPi's audio jack to aux input in car or direct output from iPod to car.
The RPi can be wireless guest when close to home or wireless AP in car. Or if myfi mobile access in car, maybe use the myfi for the AP for everything.
