We're attempting to install a 12v USB charger, tapping into the kill switch wires (because they're accessible). Thing is, both wires are black, both have writing on the wire casing and there are no distinguishing marks on the terminals. While manually depressing the switch, we've tried 3 different types of volt meters and get no readings at all from any of them. Lights are working when the switch is depressed so we know the circuit is live. How can we determine which wire is hot? What are we missing?