I have the same problem on my 1983 300SD. The drivers lock does not really turn. It only turns a little bit and will sometimes get stuck. I'm going to remove the panel tomorrow to try and rehab the lock. Does anyone know how I can remove it if I am unable to turn the key in the lock and without damaging anything?

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W126 1983 300SD 286,000 miles and ticking
Baby blue exterior Grey MB tex
Recent work:
Replaced air cleaner mounting brackets and heat shields
Replaced alternator, fan and power steering belts
Replaced positive battery terminal
Replaced negative battery terminal and cord
New Duralast Battery

My car needs work.