[VetteVet]

According to the Haynes wiring diagram all the gauges are fed 12 volts by a pink wire and the other terminal is connected to a variable resister to ground.
The variable resister being the temp sender, oil sender (electrical) and the fuel float sender. It seems to me that if you apply 12 volts to the plus side of the guage and ground the - side you should get full movement of the needle to the left and if you add in resistance you should get a reading of some sort on the gauge. I have a couple of instrument clusters from a 68 and a 70 that I can test this theory on and let you know. I think I remember that the temp sender was 75 ohms. I'll measure one tomorrow now that you have my curiousity aroused.