Unplug them one at a time and see what stops working..................
Trace them back until you know you are following factory wire colors and then look up the wire colors in the manual
Has any equipment, driving lights, fans, stereo, theft alarm been added?
If not, check for high beams, low beams, ignition system, charging system
Looks to me like a lot of choppin' goin' on. Personally, I'd plan on a rewire.
ONE BIG PROBLEM with many many of these old girls is poor connections through the bulkhead connector.