I have a 1973 so I am not sure that your set up is exactly the same as mine.
Having said that, I believe up under the passenger side you will see a bank of connector blocks. Your wire harness is separated into a few pieces. If memory serves me there are 4 larger connectors with 10 pins in each. There is a also a few individual connectors. The 4 larger connector blocks (black white green and blue) connect the dashboard or instrument harness, hatch area harness, to the engine harness. The relays for things like the horns, and wipers are located on the passenger kick panel and are connected with individual connectors. There is also an “L” shaped connector that connects the hazard relay. Another for the main power from the battery to the fuse block. Can’t remember other detail at this moment.
Anyway, I would start by taking copious notes and labeling the various individual connectors, and I would disconnect the battery and leave it disconnected until the issue is resolved. Write down the order of the 4 larger connectors, they are colored and I believe keyed. I think there is a good chance that the problem is one of the relays, but that is just a guess from some of the things that you posted. Don't conclude anything based on guesses. You can troubleshoot this to the point where you can eliminate the actual issue, and not waste time or money replacing items that are not damaged.
After labeling all of the connectors I would disconnect them from the engine harness and see if the short still exists. If it is gone than reconnect one connector at a time to determine which harness has the issue. If the short is not removed then I would disconnect the individual connectors. If everything is disconnected and you still have an issue then the problem is likely to be in the engine compartment. I say likely because there is always the possibility that a previous owner has added or modified something.
Once you figure out which area has the issue then I would get a schematic and track it down. I have a laminated schematic in the garage and will post a picture when I can, a bit later today. If you do not have a meter or short detector you need to buy one, you can get something fairly inexpensive.
You may be overwhelmed, frustrated and confused right now. Electrical systems are similar to plumbing, without some of the sloppy mess. Go slow, take your time do one thing at a time and draw a conclusion. If your not sure post the question. Write down what you are doing and what the results are so the forum can help you. It may take a while but you will get through it.