[patrickt]

It almost has to be a bad ground path for the bulb in the gauge. Instead of the bulb having a nice, clean ground path back to the negative terminal of the battery, the electrons are taking a "short cut" across the workings of the tachometer, and the result is that the tachometer doesn't work. Try running a wire, with alligator clips on both ends, from the negative side of your battery and clamp it on a metal portion of the gauge (try a couple spots) and see if that temporarily fixes it. And if there's a nice loose ground wire running to the gauge, try jumpering to that as well. Sometimes the "jiggle" test is your best friend.