Engine/Transmission (1994 - 1998) engine coolant temp gauge

[twhalley]

Engine coolant temp gauge stopped indicating around 50K miles. Erratic indications for about a week then nothing. I replaced temp sender unit and all was fine for about 100 miles, then same story. While on the road stopped at a 5 star Dodge dealer to have the frayed throttle cable replaced(one strand left), I asked them to fix the inop temp gauge. Their diagnosis was a stuck open thermostat. They replaced and it barely lasted10 miles and since I was on the road and it was Sunday, I did not go back. I installed an auxiliary temp gauge from a tee in the heater hose to engine block. Not the best reading but it works. On the road to the next dealer, who says temp sender is the problem, which they don't stock. Back on the road, order another sender from Cummins, half the price of the Dodge part but same Cummins number. When home install 3rd sender unit. Same story after 75- 100 miles it quits. If the wire at the sender to the gauge is grounded it goes full scale deflection. So gauge seems ok. I suspect a hot ground or chaffed wire(none visible) as best can been seen or touched. Any suggestions if ground problem, how to check and isolate??