OK good progress.
So, 3 is 12V+ power for the gauge, 7 is ground, and 6 controls the temp gauge needle. High resistance between 6 and ground is cool temperature, low resistance between 6 and ground is hot.
You could probably use an ohm meter to measure how many ohms your temperature sender gives you between ground and the center pin of the sender. Then put the sender wire on it and see if you get a similar number of ohms inside the cab at the connector while it is disconnected from the instrument cluster. Maybe your sender wire has a bad connection or high resistance. Or something wrong with way the temperature gauge is connected to the printed circuit.
Member davepl showed us this chart in
this thread, it is how many ohms you get from the sender at various temperatures of the engine.
