Mercedes 300TE electric problems, part II

A couple of weeks back I mentioned that my newly aquired Merc 300TE had a lhs rear light that was playing up in the most interesting fashion - it seemed to have a live feed to the tail light even though the
ignition was off, plus it appeared to be suffering from grounding problems at the rear light clusters.
After popping a couple of fuses in order to find out which circuits didn't want to play, I managed to improve the situation - the rhs rear light was now working properly, whereas the lhs tail light wasn't working at all. On top of that the rear interior lights had stopped working, too. Great improvement, that.
Today I finally managed to spend some time trying to trace the fault, only to find that part of the rear wiring loom (lhs, the one that goes over the inner wing to the rear light cluster and into the roof) had gone into proper meltdown mode. The loom looked a bit 'shrink wrapped' from the outside so I carefully cut it open. Took me a while to realise that the cables weren't glued together, it was the melted insulation on one cable that supplied the glue. Great. Looks like the damage is a little bit too extensive to repair unless I call in a pro as it looks like I'd need to cut out about five inches from the loom and replace all the cables. And that won't bring me any closer to finding out as to why it melted in the first place.
I guess I could try to find a secondhand loom and just replace it (would that be the time when someone tells me that this is actually the main rear loom and that I'd need to disassemble half the car just to get it in and out?). That, or stick it on fleabay as "spares or repair" and get myself another one...
Oh well, I guess my pot's half full...
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that won't bring me any closer to finding out as to why it melted
Either defective insulation (unlikely as two wires inside the wrap would have to had defective insulation) or overheated wires from a short (unlikely, that the fuse would not first blow) or the harness was damaged in a wreck.
I'd excise the defective section and create a patch from new wires and splice the patch into the existing harness and carefully tape it up.
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