sure this wont help your precise problem, but i had a problem recently with the rear lights.
the OS worked as it should, but the NS wouldn't work a tail light, yet would still light up wen braking (less bright than the OS if sidelights were off/the same if sidelights were on). After checking the bulbs and fuses, i got frustrated and started taking apart every light on the rear of the car.
turned out that the number plate lamp was a rusty mess inside, and only one of the two bulbs in there was working. so i fiddled with it a bit and problem fixed. £5 later for a new unit on there and job done.
just goes to show how wen it comes to auto electrics (esp ona defender) the problem mite be something completely unrelated.