85 Chevy Silverado: How to test temperature gauge?

I have an 85 Chevey Silverado. recently I had to replace the instrument cluster housing because of a bad connection with the gas gauge. Now my Temperature gauge will not work, and I am at a loss for how to test it. I did manage to replace the gauge with another one from the same type vehicle and this did not work either. I tested the two main leads to the right and left and together they are bringing in a strong 12 volts but I am at a loss for how to test the top lead as I believe this is the one that sends the voltage that alters the gauge itself. Any help is appreciated. Thanks.