... " So first, the fuel gauge: the wiring diagram shows three connections on the gauge, one to the ignition and two to the sender in the tank. I have no idea what sender is fitted now, but it was working with an aftermarket fuel gauge and the vehicle is now 12V negative earth. Can I get away with the original gauge? How do I wire it? " ... You can try an inexpensive voltage reducer from 12 to 6 volts on the hot wire ( the wire to the ignition ) that goes to the gauge and see what happens . As you say there is no telling what sender is in the tank .