The intake must come back off this weekend. I tried to order a new part to replace all of the wiring from the glow plug relay out to the glow plugs - I was told this harness can only be ordered from Germany, where, for $150 they will make me a new one and ship it to me in about a month. Clearly, this won't do--so I'm going to take the harness apart this weekend and see if I can find the short and repair.

Any idea on what would make a good conduit to prevent my repaired wiring from succumbing to engine heat?

I was able to "rebuid" that entire harness on my '95.

Each connector is designed so the wire can be replaced - I purchased a few spools of appropriate gauge wire (10ga as I recall) in different colors - and re built the harness one wire at a time.

I would think the commercially available "wire loom" the corrugated stuff with the slit in it would be fine...

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95 E300 Diesel 265k and counting

You are going to want to use multi-strand, not conduit. That stuff is for AC, it's hard to work with in car applications too. I don't know much about heat tolerances though.

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My Primary Driver - '85 300CD - 4-speed conversion, 2.47 rear, lowered, euro headlights, rebuilding (not restoring so much)

Wife's - '08 Saab Sportcombi Aero

Riding a '03 Yamaha Warrior

You can buy wire loom at a good parts house. Conduit or loom will not protect your wires from heat. Use wire with a suitable heat rating and dress the wire loom away from the heat source.