In your case, the clicking, and it's the same with the wires unhooked, it's pointing to either a contactor whose contactor points are not electrically closing, or possibly significant contact resistance in the harness.
Get a voltmeter and check to see if your plugs are getting power to the bullet connector. They should be getting about nine volts assuming everything else is healthy. You will get more of some plugs are open or have bad connection. If you don't get any juice at the plugs, check the two large terminals on the contactor. One should have a large ( 6 gauge or so ) wire going to the battery. Hot at all times. Probably has a fusible link in it. Be careful around it. High current. Won't shock but sure can get hot really fast. It should have battery voltage on it at all times. Key position doesn't matter.
If you have voltage, measure the other big terminal connected to that spring-like structure ( it's a current limiting resistor that feeds the glow plugs rail ). Try to start. That entire resistor assembly should have voltage on it during start attempts when the glow plugs are supposed to be on.
If you have voltage at one pole, but not at the other, when the coil has clicked and the glow plugs are supposed to be on, then your relay has open contacts. To verify, short between the two big poles. No one else. Just the big poles. With a screwdriver and see if that brings the glow plug rail up. You can bring the truck up by manually goosing the glow plug power while a friend operates the key start. Do not hold for more than 15 seconds or so. This assumes your plugs are still good, and your contactor has bad contacts.
At least this may get you started.
This is risky. Be careful as usual and stay clear of moving things.
I had nothing but very expensive problems with supposedly factory replacement glow plug controllers.
The newer ones would quickly burn out brand new sets of glow plugs.
It appears to me that slowly but surely we are losing the skills to build things as junk sells for the same price old-school quality sells for.
I traced my situation to defective contactors that failed to open after several seemingly normal operations.
This happened twice to me. My woe started upon the replacement of the original 30 year old contactor that finally died of eroded contacts. It simply wouldn't close the circuit anymore despite going through all the motions and noises of doing so.
Replacement of the contactor was simple enough. The new contactor failed within a week. This time, shorted. Welded contacts. Would not turn off. Even if key off. Destroyed my alternator too. That piece of crap part cost me several hundred dollars and several days of chasing wild goose.
I bit the bullet and bought a brand new controller ( which came with another brand new contactor in it ).
Much to my frustration, it, too, failed within a week! Same friggin issue! Contacts welded shut. Another round of wild goose looking for unintentional glow plugs to ground shorts, monitoring currents, and watching ammeter monitors like a hawk. Nothing. Even got a battery powered oscilloscope to help if I had some sort of thermal short in one of the plugs ( that resistor strap on the contactor post makes a handy shunt resistor for monitoring current. It drops about 3 volts when fully loaded with good plugs ( 200 amperes. Eight plugs. 25 amperes each )). I was looking for spikes above 3 volts. None found.
CDD is a member here who is well aware of this fiasco in the parts supply chain. He gathers up the appropriate connectors and wire and kits up glow plug harness replacement assemblies. They do have a tendency to corrode as they carry 25 amperes per glow plug, just held in place by bullet connectors. The connector gets corroded, then it heats up, then things go downhill from there.
The wires and connectors were not my woe. I cleaned those up and squeezed the connectors a bit until they fit snugly on the plug. My woe was that friggin' relay! Wes sells a kit to convert the glow plugs control to a manual pushbutton ( or momentary spring-return toggle switch ) using a White-Rodgers industrial grade power contactor that has a 12 volt coil.
I believe Emerson Electric is now making them. I understand they were designed for large industrial HVAC controllers. They run about $100 to $200 /ea ( about 10x the cost of the auto parts store parts price. ). For me, well worth it as if these things fail, there are substantial cascade failure costs and lost time in goose chases
That is what I am now using. I have had absolutely no further issues with it. It is not mechanically the same size as the original relay ( It is larger and heavier. ), so you may have to rearrange some mountings to make it fit. There are many glow plug woes and cures posted in the IDI forum here. Search for them. Some of them are mine. Several years old now.
Few things are more frustrating than finicky machines when you can't trust a supposedly new part to be good.
