Okay.... finally got around to working on my truck.
I ran a new wire straight to the connector that plugs into the gauge cluster, and now it does nothing.
So I decided to test the gauge itself to see if that was the problem. Doesn't appear to be.... but here's what I did: I physically took the gauge out of the cluster, started the truck and got it to operating temperature (10 minutes or so, and drove it out of my garage up to the top of the hill. Felt the hoses and they were at operating temp). I used alligator clips to connect the gauge directly up to the battery, and then another alligator clip from the temp sensor plug to the input on the gauge (they are labeled on the new one I bought). I tested all three gauges that I have, and none of them budged. I know the needles are functional because I noticed that before I hooked up the ground, the needle slowly pegged out to hot, but when I clipped on the ground, it went back down to cold.
I replaced the sensor, but I guess it could be a defect, what do y'all think?