Right hand tail light not working

On my 1999 110 TD5 Defender, my right-hand right hand tail light has stopped working. The bulb was blown but replacing it has not fixed the problem.
All other lights work, including the right-hand brake light and right-hand side light so that rules out a fuse or earth problem.
I measured the voltage in both the bulb holder and the plug (with the bulb holder removed), on the red wire I'm getting 0.7 volts with the lights on.
I read an old post on here from 2011 that suggested the wire shorting on the body behind the dirt flap under the wheel arch where the loom enters the rear of the body. Looking here, there are no obvious signs, all wires have the black covering on.
Is this area the most likely cause of the problem or is there anything else I could try? Also is there a safe short-term workaround as the MOT is due soon - can I run a wire from the left hand tail light or would that overload the LH wire? In case I can't fix it.

you could run a fresh wire to it if you cut the old feed to holder ,but it sounds like theres a poor connection or damage to the feed wire to rhs tail lamp

That's what I was thinking. I think it's going to be easier to run a fresh wire to the holder than to try to trace the fault (possibly within the loom). If I was to do this though, where is the best place to run it from? I don't really like the idea of running it from the LH tail lamp as this already has the number plate light running from the same wire so there will be quite a lot of current - alright to get through the MOT but not as a permanent fix.
According to the wiring diagram, the wire goes back to Header 0290 (K109), connection 2 - is this the best place to run the wire to? If so, where is this header? There also seems to be a connector (maybe a bullet connector) between the tail lamp and the header, where would this be?

tail lights are low current and if you had a tralier lights would come from the same wires,brake lights and indicators higher ie 5w tail light 21w indicator, though you will be better sorting the correct wire after mot

I'll run one from the left then for now. Do you know where the header is?