With the key in run the gauge shoots up to max then falls down to 0. So, it would seem power is getting to it at least. I think it's something in the engine bay not hooked up right if at all.

Check it out: there's a wiring harness that looks like it's coming from the dash because there's a ton of wires, then it seperates on the drivers side down to two wires - one purple heading to the oxygen sensor, and a green wire with a connecter. The green wire's connectors plastic housing is fried, and it's loose just hanging around. What's the green wire connect to, and is where can I find the temperature sending unit? Any advice?

 

The green wire is the wire for the temp gauge. the sending unit should be in the head.

 

Seems most logical, unhooked wire, gauge doesn't work, but how is a flat connector supposed to hook up to the sending unit whose connection is like this? I'm pretty sure the green wire was unhooked since I bought the car too. Anyways, here's the pic of the wire and the sending unit....

I apologize for the poor quality, it's taken off of a webcam shooter thing =P

 

Mine has a u shape in the plastic along one of the sides and it slides over the end of the unit and snaps into place. It took me a few minutes to figure that out when I was building my s10.

 

Gotcha! Must be missing that piece, I'll go check out the autoparts store see what I can find. Thanks.