It's NOT a short in your doorjam switches. Your switches work by "shorting" to ground when the door opens, at least if they're like every other door switch I know, including on my '78. In other words, no matter how your switches might short to ground, all that would happen would be that the dome light would come on.
What I see in my '78 schematics is that these things (voltmeter, dome light, map light, dash lights) are all powered through different fuses. I'm sure it's the same with your '76. I'm guessing someone did some creative wiring "repairs." Even so, these things all seem to run through a fuse, which is good. Not all professionals are so courteous as to make sure their ad hoc repairs are fused.
Based on what you've described, I'm going to guess that there's a short in your dome light -- perhaps in the socket, or perhaps in the wiring that connects to the unit. When you open the door, the door switch grounds the dome lamp unit, which is shorted, such that current runs through the 10A fuse, through the dome lamp assembly (bypassing the bulb), through the door switch, and straight to ground. This of course would blow the fuse. I'd almost bet money on it!
Please let me know if I got it right.
Peace,
Sarah
EDIT: I just noticed the title, which includes parking lights and running lights. Those should be on an entirely different circuit -- and in fact the same circuit as the headlights. You might be having a wiring harness meltdown -- probably from some of the creative electrical "repairs" that have been made to the cars. You might want to start untaping the wiring harnesses to search for melted wiring. Insulate any melty wires with electrical tape. Melted wiring is like a cancer and can take down other circuits too. Honestly, from all you've described, you probably need to get your car to someone who can restore your electric to OEM design and condition. Unfortunately very few people know electric.
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