While I agree that the MONO switch issue is not typical of the power harness, I would still reseat as just maybe its a current or voltage sag to that circuit board that might cause the relay to not hold(not sure if its hold state is stereo or mono). In any case, its a 20 minute routine that would not hurt anything as long as you are careful to not hit anything with static(ground yourself to a large metal object or use a wrist strap that computer builders use to avoid static discharging to whatever you are working on
