I'm with MR. I'd be checking the size of the cable to the rear or the earths.
Re-running it in a larger gauge cable (6 sqmm or thereabouts) will probably fix your problem.
If the wiring is as crappy as you suggest, I'd be ripping it out and starting again anyway. At least you know it will be right then. Plenty of folks on here to talk you through it.
Sounds like you've got a temp fix to allow you to get away which is cool.
All the best and merry Christmas!
Cheers,
Drew.


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