Hello hevail; I you have checked the grounds, you will have to start checking the positive wires for a short to ground or open. Isolate each circuit and test, and if possible, test each section of a circuit if it can be disconnected at a connector. You can remove power and test continuity from end to end, and continuity to ground, and then check with power applied and see if it is dropping voltage. Most of the time a problem will occur where a harness is rubbing/chaffing or exposed to abuse. My guess would be somewhere in the tailgate hinge area where flexing occurs.
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'94 940 150k, '86 240 170k, '72 142 KIA, '70 144 KIA, '69 144 RIP