All of the suggestions are good except do not use graphite.
Where are you and how cold is it. I had a bad window dew wiper allowing water into the lock assembly. Below freezing, I had the same symptoms.

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/mercedes-benz-cars-sale/296386-fs-1-owner-83-mb-300d-turbo-rebuild-parts.html