Yikes water and electronics makes me cringe.
Caution given to any and all who attempt to use NAOH (sodium hydroxide) as a cleaner. Acid gloves and goggles probably a good idea. Highly caustic and can cause burns at these concentrations.
That said I have never cleaned a harness so I have nothing to add except these cleaners do what they do due to their naturally aggressive nature. Do not forget to work SAFELY.
Have fun play pinball