300SD still wont start

The clue to the problem is the time element. Starter motors that are defective when hot - 30+ miles use - work again after cooling down.

Next time rap the starter and it may spring into action. That will be the confirmation that it ought to be replaced.

When you say you turn the key and absolutely nothing happens do you mean the glow plug light does not come on?

glow plug light does come on, but no noise of any kind, no turn or click

Hmm.... usually a starter will make *some* noise if it's not starting elthough complete and utter failure is not unknown.

I'd suss out that neutral safety switch first though. Bypass is with a soldered connection and see if that has any effect.

I would suspect the starter too... but can do quick test to verify if it is the starter... with a remote start... you can hook up positive to battery and the other wire to the purple wire by the passenger side engine bay where you see strip of wires into a strip connector.

Another possibility is bad ignition switch... does it have a strong return from trying to crank? Does it feels funny?

The last possibility is bad overload relay... but this should not prevent you from cranking... it will however prevent you from starting on gas engine... not sure on diesel.