Strange electrical issues with my '72 Dodge truck

Makes sense, I'm not sure if that's how it's wired though. As far as I could see that part of the wiring is still factory. I need to get a FSM for this truck, I got a Chilton's manual and of course it's basically useless, the wiring diagrams in there didn't tell me a whole lot.
Yeah those Chilton's are barely worth using to start a bonfire. I can't recall seeing any digital versions of the FSM's for those trucks but if you do find a source, please share!