just about all glow plug circuits i've ever dealt with use a relay and a timer...
turn on the ignition and wait 5 to 10 secs approximatly and also should be some kind of indicator light on the dash...
when the glow plugs are in heatup mode the light will be on....
when the light goes out then the engine is ready to start

now if this is an older glow start system it may have a glow plug in the intake and a fuel line running to it
it actually pumps fuel into the intake and ignites it to heat up the intake chamber.
would have to be a really old truck though... havn't seen those in years.