At this point the ground does not matter. It's the very small black wire in the center of the control connector. It, the ground, works because you get a GP light and relay clicks, at least sometimes, and you testing the ground isn't going to help you fix it. (It takes a wiggle test and following each and every wire of the 3 in the diagram...)
You can do two things at this point:
1) Most likely at the time: Throw a GP relay at it. The smoke and code from the ECM indicate it's an intermittent contact: doesn't work all the time.
2) If a part doesn't fix it take it to an automotive electrical shop or friend who knows Trons. For an Intermittent connection somewhere. I suggest this because IF it isn't a bad GP relay you have an intermittent connection that is beyond your ability to track down at this point. Education on this truck will be expensive as seen in burned out starters, dead batteries, broken engine destroying glow plugs when you bypass the relay and instant glow plug and engine death via the use of ether starting fluid.
You can put self limiting glow plugs in: all 8 including the real bastard ones to get to on the passenger side. Then you can safely bypass the GP system without non-self limiting plugs breaking off in the engine.
Given the risk of damage glow plugs can do to an engine I suggest you ask yourself how much frustration and cost you want to throw at your education of electronics. A GP relay is a safe slam dunk and I feel you have done what you can to make sure before throwing a part at it.
