I guess I have go be the devils advocate. I don't think you need the Glow Plug Light at all.

If you have trouble starting you know it should start OK you know need to check the Glow Plugs.

The Push Button Switch and Ford Type Starter Solenoid that subs for a Glow Plug Relay has worked without a hitch since around 1992 and no Glow Plug Light although the stock set-up had a GP Light.

If My Stock Mercedes Relay goes out I am replacing it with a Ford Starter Solenoid. But, this time I think I would wire it so the Solenoid activates in the Pre-glow position instead of a separate Switch.

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84 300D, 82 Volvo 244Gl Diesel