Reverse Light Switch strange wiring?

Hi, I've got a Defender TD5 2000 pickup that I've had for around 6 years now. The reverse light never worked but recently with a bit of cleaning up I managed to sort it. Then all of a sudden it stopped working again. Anyway decided to get a new light unit as the original was a bit tired but after fitting, it still didn't work. Thought it might be the switch on the gearbox but this look OK. This is when I noticed the wire colours on the harness side of the reverse light switch connector. Instead of Green and Green/Brown as the wiring diagram suggests they are Black and Black/White. Plus the Black wire goes straight to earth (as you'd expect by the colour). I have checked that I haven't confused the switch with the Diff Lock switch.
Now I may just have a short somewhere but can anyone explain the wire colours?

It may be the low range switch. The ECU takes a signal from the switch and changes the pedal map (and possibly a few other things) to improve driveability in low range. It's switched to ground, and as black is ground that makes sense. From memory the low range switch is on the right of the transfer box near the bottom, while the reverse switch is higher up amd on the main gearbox.