1995 GMC 2500C diesel won't start

Periodically, for several months, either the truck wouldn’t start or would shut down with no warning while diving. Now, it will not start at all. The symptom of the starting issue: the “wait to start” light would come on, but flicker off immediately. Until recently, the truck would start when the light remained on for several seconds. Now, I cannot get it to remain on and start. It is not the glow plugs or glow plug controller. Both have been tested or changed. I did replace the main ground wire to the starting motor. What should I look at next?

I would start with the fuel system. When a diesel is running the only way to turn it off is to turn off the fuel. It could be a bad fuel pump,fuel filter or whatever controls the fuel on your truck. I would bet its what controls the fuel.

It’s not the fuel pump, filter or injector pump

The glow-plug light is not acting normally. Find out why…How many miles on this vehicle? Compression test?