Make sure it is a momentary push button. If you heat the plugs more than 8 seconds they will burn and could turn into a ballon on the tip and fall into the cylinder hitting the pistons into the head as there is no clearance for a glow plug tip. My thoughts........ After 20 years of sitting pretty sure your injection pump is gummed up. Tell me about the rest of the truck. Rust free , 4wd? How does it idle and accel? Did radiator shop come up with a replacement?