12V on one side of the fuel gauge and other side, wire to a wire wound variable resistor ( sender unit ). Resistance range ~ 12-22 ohms ( full ) to
~240-290 ohms ( empty ). Sender unit should be grounded - lugged wire mounted from a sender unit screw to body.
Good point, Brett. 10 V voltage stabilizer on the instrument panel for the fuel and temp gauge. So, actually ~10 V on one side of the fuel gauge terminal
instead of 12 V. Either way the gauge operated when touching the wire to the sender unit was grounded.