66 Dart gas Gauge

If you have an original instrument cluster (I am referring to a early 70s model, not sure what year you have) the two screws coming out the back of the gauge get connected to the PCB when you tighten down the nuts. One is connected to the +12vdc and the other goes to the sending unit. There really should be no wires involved, just the traces on the PCB. The circuit is grounded at the sending unit. There is no need for a ground at the gauge.

Do you have a working set of schematics? If you do electrical work, you really need a set