What Engine does it Have?
If it is like the Diesel Car Engines of that time if one Glow Plug is bad none of them will work.
Trace Wire and find the Fuse and see if that is OK.
What type of tests have you done on the Glow Plug System?
The Zig-Zag resistance Wires cannot touch the Block or anything else besides the Terminals they are supposed to go to. The Wires and Resistance Wires have to be on the GPs in the correct order with the Insulators properly installed.
DIY Repair Links
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PeachPartsWiki: Do It Yourself Articles - Mercedes Vehicles