94 K2500 6.5 Glow Plugs Inop

94 K2500 6.5L Diesel 250k. The dash light and glow plugs are inop on my buddys truck, although the light will come on and the truck will start right up on "rare" ocassions. This has been going on for a couple of months now and getting worse. I won't get to look at it until this Tues, but I was wondering if any of you diesel guys could give me some pointers on where to look.....Steve