If you are willing to pull the cluster: Make a thing with two C-Cell batteries and wires on each end and power up each gauge. They should read about 1/2 way. That will at least tell you if the gauges toasted.

If they work, then the next thing might be the voltage regulator on the back of the cluster. Although it might be easier to replace this first since you don't have to completely remove the cluster to do so.

Although - my first guess would be a ground problem, but i sounds like you tested for that already.

If its a new cluster - did you know if the gauges even worked at all? Did you clean up the circuit board and test for continuity on the green connections on the board and make sure they were good (conducted, and not grounded)?

edit - and I'll add that you have to do the continuity tests on the board itself that I described earlier. Also - are you sure the voltage regulator is good?