The rear hatch defogger on my '84 wouldn't work, and whenever I would push the defogger switch, my gauges would go nuts, and the indicator light on the switch wouldn't stay on. Replaced the switch/delay module, and still the same problem. Happened to notice that when the hatch was up, the switch's indicator light worked. Turns out that the wire that feeds voltage to the defogger (left side of the closer) was almost completely disintegrated, probably from age. Unplugged it, cut it, cleaned it, tinned what was left with solder, and soldered in a piece of 12-gauge stranded wire. Removed the insulation from the new wire so it would be more flexible when the hatch raises. Now it works just fine. Was a real problem to track down, but maybe the information will help someone else.

 

Welcome to the forum and thanks for sharing!