I've had the wiring diagram out
There is a plug at the tail-lights (obviously), a plug drivers side top front of boot (this may be out of sight inside the guard, I'm not real sure), the plug at the front which I mentioned before and then the plug at the switch on the gearbox
I cant find what fuse it is tee-ed off, but there are a fair few things powered off the same circuit like the de-fogger relay (rear window) so if thats working I dare say the power source is O.K.
Basically just need to test the switch, or bridge out the plug to eliminate the switch completely. Get the lights working without the switch and then you can see if the switch works. If you have power at the switch harness, then the earth is at the tail lights. If you have all the other lights + number plate lights then Earth is most likely not your problem, the Earth is common to all of them.