Time to remove and repair, possibly replace, under dash harness

get yourself a few rolls of harness wire and lots of solder, terminals and shrink wrap. Pull the entire thing out and just start one wire at a time. Cut out old one, measure and cut a new one and graft in. Youll probably find there are 2 or 3 areas that are the problem and they can be "strung by" easily without messing with the terminated ends. The bulk wire doesnt even have to be 12 different colors, just get an assortment of colored small zip ties in different colors (variety tube from HF) and put a different tie or 2 on each wire where they run parallel to each other. They may all be black or red or green (Id recommend at least 2 colors of wire to start) but youll know where they all go by matching up the zip tie colors. AWM or SXL wire is auto grade. there are a few that need to be larger guage but most can be like 14.