Grounding the instrument voltage regulator

As Fishy stated Mike the panel is chassis grounded meaning the screws carry the ground to the dash frame , if you look at the circuit boards the guages are mounted to you will see copper around all the mounting point screws and holes tuck a wire under one or two of these screws and connect it/ them to the dash frame to create the complete ground path , the pot metal of the cluster acts as part of the ground circuit that's why there is no obvious ground lead . Hard to say whether you fried the gauge or the regulator you will probably have to try a known to work regulator from one of your other cars in place of this one to test that .