I am looking to hook up my backup camera easily to the wires in the hatch. All i need is a ground and 12v power. I am running a wireless backup camera, thats why my wires are not going thru the whole car.
Test light should get you the hot 12V and that bolt that is rusted could be a good ground possibly. That is how I would start looking with out a wiring diagram.
But you might want the camera to be on a keyed circuit (only hot when the key is turned to ACC) or you might kill you battery.