Unplug the green wire from the sending unit and with the key on, ground it. The temp gauge should read all the way hot. If you just unplug that green wire, with the key on, the temp should read all the way cold.
If it doesn't do that, you have only two wires to deal with. The green wire, which comes in at pin number 13 on the white cluster connector, or the pink wire which comes in at pin number 5 of the cluster connector.
With an Ohm meter, check the green wire for continuity. If the Ohm meter indicates a break (no circuit) in the green wire, replace the green wire or find the break and fix it.
Check pin number 5 for 12 volt with the key on. If your oil gauge works, you can skip this step, because the oil and temp gauge share the circuit.
If the green wire checks out, and if you have 12 volt on the pink wire (or if your oil gauge works), you have a break in the laminated plastic circuit on the back of the cluster. You'll have to pull the cluster to fix it.
I have a paper that describes how to test the gauges as well as pull the cluster. PM me if you want a copy.
It's almost never the gauge.