Ahhh thank you sir, i will cherish this forever... just waiting on my fuse to come from amazon and ill put this to action. thanks

With this circuit the solenoid is unprotected and becomes a potential fire hazard in the event of a short in the wiring or solenoid. But if you go to these lengths why not fuse the glow plugs individually this would be useful in testing the glow plugs individually for shorts or ohms. Not only would it be a safer design but useful in diagnosing glow issues down the road.

I am not sure which part of the Solenoid you mean but a Person is free to add as many Fuses or Circuit Breakers as they feel the need for.

I have no Fuse on the Manual setup I have on My Volvo since about 1992 with zero issues.

On the Mercedes Glow Plug Relay you will see a Flat Copper Wire. That Wire has a direct Connection to the Batter and as far as I know there is no Fuse in that circuit.

Notice that you have no Fuse from the Battery Cable to the Starter Motor and none between the Alternator and the + Wires on the Starter.

A Friend of mine where I used to work dropped a big Screwdriver down to the Starter and Started the Starter Cranking but not engaged. The event did not stop till the Screwdriver melted and the Starter was toast.

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There is a fuse... in the relay. Nothing before it...

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My point exactly. As a requirement to meet electrical code all powered devices require fusing. The solenoid is not fused in the circuit diagram.

I'm wondering why no one here has tried a solid state version.

Maybe too high tech, maybe cost once you add the required heat sink, maybe overall size due to the heat sink requirements or maybe availability. It would need to handle around 80 amps. It would be good for a million plus starts but most are only rated to work down to -20 degrees C. Just when I need it most! I suppose you could add a heater to preheat the solid state relay before you glow. So I'll throw in complexity.