OK so I got it working by changing the reverse trigger from the HU to the reverse wire in the harness under the kick panel. Everything works correctly now, cam comes on when gear changes and turns off when switched out of reverse BUT I still have a problem!
A few seconds after the unit turns on (10-15?) the camera switches on by its self but there is no image because I'm not in reverse. I then have to hit the home button to get out of the black caution screen.
The head unit is set to battery. The positive and ground from the camera are connected directly to the tail light positive and ground wires at the bulb. The reverse trigger at the HU goes to the reverse wire in the kick panel. WHAT AM I MISSING!!???
Thanks for the help though!