Electrical help needed, underdash wiring is a disaster

Ugh, that sounds like a mess. You can either pick up a good used harness or rewire yours. It's difficult with the dash in the way, but the entire dash was designed to be installed easily when originally built. You can take the entire dash assembly out, and work on it from the back side.

Fix just one circuit at a time, and address the bulkhead/amp gauge/charging circuit issue while you are in there. Take pictures of the before and after stages for reference.