Great idea, but remember: Operating a vehicle while using your iPad for anything other than navigation (e.g., watching videos, TV, browsing the web, etc.) is illegal in most (if not all) states.
And on the AUX input for sound: If you drive a Ford, Mercury, or Lincoln product with the SYNC system, just connect via Bluetooth and all your audio (from videos, music, audiobooks, whatever) streams to the car stereo without any wires.