Im actually having the same problem with my car currently. I posted last week about it but it seems this thread is getting more attention so i figured id jump in here too.
Did you have one start that was slow and sluggish, almost like the battery was dying as it cranked? Thats when i noticed my problem first. Then the story is the same as yours. No clicks, no sounds. no start. Ive replaced my battery, and both cables. Headlights just barely dim when attempting to start. Ive also checked the ground strap. Is there only one on a 300d?

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1980 300D