Im not exactly sure how the glow plug system works on your car, but its gotta be somthing to do with the thermo switch that tells the relay when the engine temp is to cold and it needs glow plugs.

Follow the signal wires from the relay and find out where it gets its signal.. Its probably somthing that looks like a temp sender. then test the switch im sure its faulty, otherwise theres a short somewhere in that harness.

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-->1971 mercedes 220D; 617 951 engine, 4 speed