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I am assuming you are like me and know nothing about diesels. The first thing I would do is go up to the diesel part of this site and do a search on glow plugs, and ask those guys if they know anything. I would also do a little searching on the web, and if I was planning on keeping the truck, I would go to helminc.com and see if they have the factory manuals. They are not cheap, but a couple of trips to the dealer will pay for them if you are willing to study them and do the work yourself.
If you can find the glow plugs, and figure out which one is #6, I would look at the wiring and such around it. If it looks ok, and looks easy to take out, I think I would swap that glow plug with another, and see if the problem moves, or stays in the same place. This may give you a clue to what is wrong.
I just looked under the hood of an 03 7.3 today. GOOD GRIEF!!!
There's no room to get your hands in anywhere near the engine!
You're going to need the service manuals to dig into your motor.
AL.