My gauge has two wires for the light. I thought that I could run one from the switch and ground the other one to the frame, but that does not work. I had to run 2 wires from the gauge, down the A-Pillar, and to the back of the switch panel where the fogs and dimmer live. Again, there are 3 wires going to the dimmer switch. I tried all combinations for them and found that using the green and blue wires did the trick.
I am not certain that this is the best way to do it, but it works for me and the dimmer is functional. I don't know if some gauges only have one wire. If so, then the gauge would have to contact the frame to ground.
P.S. I did not cut the wires to the dimmer. I stripped a roughly 1/4 inch section and then wove in the new wires and taped them.