1999 Ford F-250 glow plug light

Check the glowplug relay on top of the engine. You should have power to bothe the large lugs when you first turn the ignition on. If you do not, check power at the small lugs (one 12v pos, the other neg). If you have power and ground on those two lugs, replace the relay. The light recieves a resistance signal from the glow plugs. The other problem you may have is where the harness for the injectors and glow plugs passes through the valve cover gasket. They have been known to melt there and not get the power to the injectors or the glow plugs. Check the relay first and then move on to the valve cover gaskets. If you have any other questions, feel free to give me a shout.