Do you own a volt/ohm meter? The sensor should chnage resistance as the truck warms up. Take the sensor you know works and touch one end to the contact and the other end touch to a known ground or the engine itself and measure the resistance (ohms). It should change between when the engine is cold and hot. If it does you know the sensor is ok and the problem is in the wiring or the guage itself.

These guages use a heated coiled strip of some type of metal and moves the pointer based on how hot/how much current it gets. They are going to move very very slowly just fyi.