Electrical connections are most likely your problem. There is a pack of wires going from the pass side inner fender over to the back of the engine, they run right over the pass side valve cover. In this wiring about in front of the A/C unit there is a plug. In this pack of wires are two large yellow wires that feed the glowplug controller. The plug can't handle the current of the two yellow wires, and over time will melt the plug. If yours is melted, cut the yellow wires on both sides, and connect them bypassing the plug. What I did was to crimp ring connectors on each yellow wire, and then use small bolts to bolt the yellow wires together. I am glad I did this, because when I had to change the valve cover gasket on that side, I had to unplug the original plugs, and unbolt my "bypass" connections to get the wires out of the way so I could get the valve cover out.
If you have already bypassed the connector or yours doesn't look too melted, another place is the connectors on the glowplugs themselves. If it short cycles like some of the glowplugs are bad, go out and unplug and plug back in all the connectors on the glowplugs,, and then try to cycle it again.
The glowplug controller works mainly on current flow through it to determine glowplug on time. If something disrupts the amount of current flowing through it(like a connection with high resistance) then it "fools" the controller into thinking it doesn't need to cycle the plugs as long as it should.
One other thing to check would be the ground to the controller itself. It's shown in the picture below.
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