I am going to raise some ire here
I took my 81 TD into a locksmith with the same problem
he sprayed some WD40 into the keyway
they went around the car and sprayed the rest of the locks

That was two or three years ago
I have not had a problem since

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69 220D not running
79 240D parts car
80 240D Beater runs everyday
81 300TD temp disabled cooling problem
82 300SD sweet