[LH Lead-Foot]

Using this forum, go the the home page, scroll down to "General Truck Forums", then 5 lines down, you will find "Electrical". Click and you will see "Color Wiring Diagrams".

This is one place to start. Using some masking tape if needed, write down and place the tape on the wires you have identified that belong and use the schematic to see where they go. The wires used on GMs have colors that match almost to the same function as that on vehicles until GM hit 1984.
At this point, GM announced in the electrical repair section of the paper manuals that colors, circuit number and the style of "Cells" of each single function for a system of device is used for every vehicle, from there forward. They have been the same ever since then.

Each "Cell" is played out with B+ at the top, fuse, connectors, switch, wires, component, wires and ground at the bottom of each page. This style is called "Marshalling" Like little soldiers marching from top to bottom.

I feel your pain when it comes to a vehicle new to me that is 50+ years old and the electrical system with / or without everyone messing with these wires.

Get familiar with the system, if you have an amp gauge on the dash, you will find two black in-line glass tube fuses under the hood that one side goes to the alternator and the other side goes to through a shunt in the amp gauge, then to the battery. These are not well understood and cause people lots of problems.
Otherwise, start with a good visual, fix battery cables, grounds to everything and the fuse box. Any additional request for help will require, what you have done, what you have, what works and what does not.

Depending on your electrical skills, check out some youtube vids for "DC Voltage Drop", "DC Series Circuits", "Testing DC Voltage", "Difference between DC voltage and Amperage" and becoming familiar with a Digital Multi Meter. Plan on a trip to HF to pick up some useful tools, like that DMM, bulb type test light (LEDs are ok, but bulb type are more useful) and perhaps a clamp on digital read-out meter that includes DC amps. Once armed with these tools, you can get a lot done. Read a wiring diagram like water in a hose. Use WD-40 or Simple Green to clean wires for correct color and be prepared to find corrosion inside the glass fuse box. There are ways to clean the connectors.

Just remember, equipment like this is better that it ever used to be, more accurate than ever and their cost have come way down. Also, these will be handy for years to come.
I spent 48+ years working on motorcycles, imports and domestic vehicles, both old and new. You do not need a $500 Fluke 87-V with lifetime warranty to fix 50 year old wires...but I did to earn a living. I have an entire SnapOn tool box with nothing but electrical tools, connectors, solder, dielectric grease, cable, connectors, $140 crimp pliers, amp clamps, terminal removal kits, etc.
You jest need to have a few of these tools on hand, then with a schematic, will help you fix anything.

Best of luck. (NIASE) Master Tech since 1978 / Retired