Defective Wire insulation
By all means , carefully slit open all the wiring harnesses that are exposed to engine heat and check the condition of the wires inside.
You should make the slit with a straight edge razor blade or aircraft hijacking weapon (excuse me box cutter), preferable on the side most exposed to the heat. A 2" long slit is plenty. pull some of the wires out of the slit to see if they are still flexible and sound.