The output of the Apple TV is HDMI, but the inputs in the Q50 are composite. The conversion is done by a mini converter originally powered by a USB port. Both the Apple TV and converter run on 12 volts supplied by the accessory power in the console.
The Apple TV runs on 3.3 volts, and the converter runs on 5 volts. This is handled by separate step down modules:
The HDMI to composite converter:
Amazon.com : ViewHD Universal HDMI to Composite / AV Video Converter, Support PAL / NTSC Standard TVs : Electronics
I stripped the cases off and put all the soldered components into a recycled Apple Time Capsule box (cut down). I'm debating changing the form factor to fit directly into the center console, or mount it in the trunk. I can also easily connect any HDMI device in the future.
To clarify, I'm using my LTE iPad to serve as a wifi hotspot for the Apple TV, and to allow me to use my iPhone to control it via the remote app. The LTE is actually faster than Comcast in many cases! This allows me to do anything I can do with the Apple TV in the living room, just now in the car.
Before I knew I was getting the Q50, I had done this for my wife's JX35 so the kids could watch streamed videos, or pictures/videos stored on our iPhones via Airplay. So even if there's no LTE coverage, they could still watch downloaded content.
Link to my first video:
