If you have an in-dash touchscreen you may as well take full advantage right? You'll probably be wanting to install your own reverse camera, and have it fully functional.
1. In the Subaru the wire tapped is used to turn on the reverse lights. Its Brown/Yellow, in the passengers kick panel, and on the top plug. You'll need to unplug all the cables to get at it.
2. Using a twisted pair RCA, one of the pair is used to power the camera. These are low amperage camera's so this small gauge is fine (check out PoE IP cameras).
3. Use a relay to turn the camera off. The trigger signal is the same you'll use to switch the headunit over automatically. The relay & camera ground can be spliced together and attached to chassis or factory wiring. Power should be from a permanant 12V source. The output connects to cable running back to the camera.
4. Zip tie the RCA to the factory harness where possible for a safe clean install.
5. Make a fishing rod from an old wire coat hanger, heavy duty zip tie or antenna.