Your biggest stumbling block is authentication. iPhones will only spit out digital/USB audio if they get the correct handshake from the device it's connected to. Car manufacturers have to pay Apple for the required authentication chips for this to work.
If you use a lightning to 30 pin adapter cable, you can still access the serial port (for control) and get analog audio. It's even possible that the car is doing this under the covers, and you're not actually getting digital audio.
The genuine Apple lightning to 30 pin adapters contain a DAC, and the required authentication chip to allow this.
Ian.