Very easy to bypass the unit. Its the same as my d2. On the main power harness there is an empty plug. You can use the cell mute wire or another wire. You put it in that hole and ground both that hole and the parking brake wire. The unit is then bypassed for video or navigation while driving. If its a used unit make sure that you reset it after you get it all done.
The light purplish wire is either the vehicle speed sense wire or the reverse wire. Reverse helps the nav know when you backing up and the speed sense helps it when you lose satalite signal. I can't remeber off the top of my head what each color they are.
If the rest of the unit is turning on and the screen not turning on try reset. If that doesn;t work then its probably gonna need service.

also try www.avic411.com experts on that unit. Make sure you use search first or they will flame you about asking the same questions.