Electrical help needed, underdash wiring is a disaster

another suggestion: gut the dash but leave the frame in the car. when you unplug the dash harness from the bulkhead and other locations, you'll be able to lay it out and inspect it. any bad wire change out. it's not too hard to do one wire at a time. just follow the wiring in the manual. while it's out, ring out the other harnesses with an ohm meter. you can probably do it in a weekend.
p.s. pull the seats and drop the steering column, it's worth it

good luck