[pcbbc]

The blue/black wire from the reversing light (which you have the red wire patched into on your first picture) needs to go to the purple/white reverse gear signal wire for the headunit (in your second picture).

What other wires do you have? How are the connected? It would probably help if you draw us a diagram.

There will be an earth wire at the headunit end in the car wiring harness going to the stereo. So if you need 0v there, then there is no need to wire a feed the whole length of the car. However normally you don't need earth at the headunit end for the camera wiring. Usually only required at the camera end, if at all. In which case 0v is the brown wire in your first diagram.