Other things to try.
Sounds like the towing electrics are a bodge job, so remove the offending wires, pull the two rear internal panels off and check for connections/ towing relay/ bad earths.
Has the battery had numerous live/earths added, if so remove them. Check the main battery earth is sound on both ends.
Check all fuses in both fuse boxes, press the reset in the engine bay (doubtful, but you never know)
And check the back of the ignition barrel, the short should not have got this far, but anything is possible.
Mike