all a glow plug is ,is a heating element, when elements fail they usually go open circuit, a short however is not unheard of.
i would remove the wiring to plug no2 and put one lead of your multimeter (set on ohms) on the plug the other to earth, my plugs read 1.2k.
if you dont own a multimeter get a 12v bulb and connect a wire from the +ve on your battery to one side of the bulb and touch the other side of your bulb to the threaded bit on the top fo the glow plug (with the glow plug wiring removed) if the bulb glows but is dim all is well, if the bulb doesnt glow at all either the plug is open circuit or the bulb is knackered, if the bulb lights normally (ie bright) the glow plug is shorting.
hope this helps.