Other than a quick check for damaged wiring, the best place to start is at the gauge, There will be three terminals on the back of the tack gauge (plus the light). One will be 12v with the key on. It will most likely be marked with a +. One will be ground and should be marked with grd, -, or a ground symbol (which I don't know how to add to the post). The third is the signal from the coil and may be marked S.
Check for 12v and ground first. If they are OK, connect an old fashion incandescent 12v test light between the ground and the signal terminal. When you start the engine the light should light up and flutter rapidly.
If you have 12v, a ground and the light lights, you have a bad gauge. If any terminal doesn't have what it needs, you're into wiring repairs.
