Richard to test which one, remove each glowplug in turn keep the wire attached, turn on the ignition touch the threaded portion of the glowplug to a good earth point and get someone to swith on the ignition and watch to see if the glowplug end glows, do this to each and if the one plug doesnt glow there is your short. Test the plug first at a good point.
Lenny Ex Reme Tank Mechanic, November 2007