Reversing camera problem

Batding

I've purchased a reversing camera for my M/H and had it fitted by an agent but they cannot get the auto function to work. I.E. The unit is a white double camera type that shows a constant rear view and should go into a close reversing mode when reverse is
selected on the gearbox and therefore picks up it's power from the reversing light but this isn't working. Has anyone fitted this type of camera and if so have you got a detailed wiring diagram that may assist me in getting mine to work. Thanks.....