Presumably one could also short the reversing wire to the fog light wire and get that to turn on with reverse? Might break a few regulations though, so perhaps fit a fuse to the short to enable it to be disconnected for MOTs etc... just a thought!
If my interpretation of the wiring loom diagram is correct the four cable connector to the existing rear fog light below the number plate, also carries the gearbox switched feed to the reversing light whether its fitted or not, so you can choose to have either. Each are fused independantly and have a dedicated return, hence four wires.
I hope eventually to re-locate the rear fog light, but in the short term a reversing light is going to be far more useful.
