Using correct terminology to not get confused, on the V6 the coolant temp sensor is for the computer and located in front of the engine. The coolant temperature sender
for the dash gauge/light is located behind/below the ignition coil. Most older cars have a gauge or a light, but the Fiero has both, so the 2-pin connector is just that, one for the gauge and one for the light. The factory connector often degrades over time and you can buy a replacement connector, Delco #PT110
To check and see if everything is working correctly...
1. Check resistance on sender terminal opposite of the cutout/notch (for the gauge needle)
it should measure 3.5 to 4.5 ohms at 68 to 72 degrees F.
2. Then put a jumper wire from the green wire (for the red dash light) to engine ground with the ignition on and the red light should illuminate.
3. Now the other terminal, put a jumper wire from the green/yellow wire (for the gauge needle) to engine ground to see if the needle pegs.
That is how everything should work.
PS: The switching of wires behind the instrument cluster is a completely different issue, that is a mod to correct a factory flaw in the original GM wiring engineering, that flaw being the voltage being routed incorrectly which causes the temp gauge to peg/slam every time you start the car, and eventually it damages the gauge. You don't have to switch wires behind dash to correct this factory flaw and prevent gauge from pegging/slamming on startup, that is a more complicated and permanent mod. There is an easier and safer way, all you have to do is disconnect/remove the double light-green wire
from the ignition switch located on top of steering column and tape up the end so it doesn't touch anything when left hanging under the steering column (I actually taped a note on mine so future owners who find it know why I did it). This bypasses the red temp light bulb test feature on startup
and the gauge will no longer peg/slam on startup and the temp light will still work, it just won't light up anymore briefly when you start the car.
[This message has been edited by Skybax (edited 09-14-2021).]