You could always upgrade to pencil style glow plugs and retrofit one of these relays. Shorter glow times and if one burns out, you can still start the car.
I was gonna do this for my friend with a 1978 300D (which is why I have them). He's not very attached to his car and selling it soon, hence they are for sale now.

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1985 190D 2.2L 717.411 5-speed
1983 300D Euro OM617.950