Quoted from LOTR_breath:Sometimes the breaks in the wires cannot be seen, because the conductor is broken inside the insulation. You can try connecting an ohm meter at each end of the wires and wiggling everything to see if you get any breaks in continuity. Another thing you can try is this. Look for and bends or crimps in the wire and give it a firm tug. If the wire inside is broken, the insulation will snap in two.
I just worked on a LOTR with this very issue, where one of the wires had actually separated inside the insulation. The switch would work just fine when the wire was loose, but as soon as it went taut, it would stretch the wire apart inside the insulation causing the switch to become flakey. Anyways, I just completely replaced both wires to the connector and switch and it solved my issue.