Reversing light wire

I apologise if this has been asked before.
We are trying to fit a reversing camera onto our year 2000 Symbol.
We've attached the black wire of the camera to the black wire on the rear lights panel, but have no idea which of the other wires is the reversing wire.
There is red/black, grey/yellow, white, black and blue. ARGHH! The manual doesn't say which is which.       
Tried attaching the red wire to the red/black to no avail and then to the white one, again nada.
Any help would be appreciated.

*Update*!! Sorted!! Through some logical thinking on Hubby's part, we now know which it is. If anyone needs to know....it's the WHITE one!
Reversing camera up and running beautifully.