1992 Chevy 3500 HD Glow plug sensor

[fisher429]

This is a 6.5 liter turbo charged diesel with a relay on back of engine to turn on glow plugs. The glow plugs were changed about a year ago. There is a indicator light on the dash to tell when the glow plugs are energized. Sometimes the plugs don't energize when the key is turned on. I think there is some kind of sensor on the engine that is supposed to turn on the plugs for some time or until they reach temperature. Could this unit be failing and where is it located? Is it difficult to change? What do I ask for at the auto supply store?

Thanks for the help