Yay - fitted and working!
I've now got an all-in-one 3rd reversing light, 4th brake light and fog light, all enhanced with waterproof connectors, shrink wrap tubing and plenty of tape. I added 2 extra in-line connectors which sit inside the boot, so I can disconnect the 4th brake light and/or the reversing light if an MOT inspector gets a bit arsey.
This photo shows the 2 wires on the R/H tail light cluster that I tapped into for the reversing light (green & white wire) and the 4th brake light (red wire).
Had one scary moment when I switched everything on - the R/H OEM reversing light didn't work. Then I remembered I'd dropped that bulb onto a ceramic tiled floor earlier in the day. Although the bulb looked ok, it was actually blown. One new bulb later and wooohooo - everything works!
NOW I NEED MORE LED's...
...Of course now I want ultra bright cool white LED bulbs for my reversing lights, to match my new one. The OEM filament bulbs look decidedly yellow. Anyone know of some good quality, very bright reversing light LED's?