look at the diagram I linked, upper right shows 2 wires to the tail - if the wires you show are under the cab, the 3rd wire will be to the gas tank sending unit - still leaves you with using an ohmmeter to track which is what, or stripping that rotten cloth off to visually determine which is which - and the wire showing voltage will be for the circuit you have live, headlight switch on, brake light switch closed, power to gas gauge ..... usually the battery should be disconnected when wiring is in that condition!!
Bill
