I wrote this guide on diagnosing the door locks on a W123, but some of the techniques may apply:

https://www.ifixit.com/Guide/Mercedes+W123+Vacuum+Lock+System+Diagnosis+Technique/20201

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

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