I know after pulling the dash out of mine and replacing the lights behind the taco ,warning lights , blinker arm, heater fan fuse wiring,ignition it is best to coat all the contact with a product like corrode cure or inox as all the contacts on mine had 31 years of dust and slight corrosion, otherwise you will be chasing stray earths everywhere, a multimeter is a must you can pick one up from supercheap for about $15.00 they work just aswell as the expensive ones espiecally only 12 volts, i have traced shocking earths in the heater fan and in the dashboard and moved the earths to more positive connections like the body with paint removed.
Hope this helps, only i know what you going through, it took me a whole day to find the broken wire in the loom for the heater.
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