My guess is they material they make these harnesses is fairly heat resistant.
I was looking at my harness the other day when I was swapping out a dying alternator. (The bearings went, and were loud enough you could hear them knock and bang over the sound of the 6.0 it was sitting on, running)
The glow plugs for cylinders 5 and 7 are right under the down pipe and turbo.......
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If you look to the right of the lifting eye, you can see the glow plug harness. The red plug the the #2 is point at, is the hot side out of the turbo, where the down pipe would be.... The reason they have the shape they do at the glow plug end, is because they actually seal the hole the glow plug is in, in the block. Much like the spark plug boots do on a 4.6L Ford Engine's coil-on-plug boot seals the plug hole....
Also, they have the connector body on the other end, because they plug directly into the Glow Plug Module, which you can see bolted to the valve cover, in front of the oil fill....