[ab54666]

I think you have/haven't done;

1. Connected the live wire from the camera to the reversing light? (the red wire)
2. You haven't connected the head unit to the reversing light? Therefore your head unit doesn't know to switch to camera.

Firstly, if you press camera on the headhunt does it all work? If so then you just need to connect the Purple/White wire from the head unti to the reversing light.