Coolant Temperature Sensor
To test the temperature gauge, short the green wire to ground and the gauge should go to max. if it does, then the gauge and wiring is ok. This would indicate that the sensor must not be working correctly.
On the 4.3 liter V6, the coolant temperature sensor is located on the passenger side of the truck, just above the manifold, between the number 2 and number 4 cylinders. Before removing the sensor you will need to drain some antifreeze, or at the vey least put something under the engine to catch any antifreeze that drains out the hole. There is antifreeze in the block, so even though I was very quick at swapping the new one for the old one, I did lose a small amount of antifreeze.