Check the plastic shifter bushings on the trans shifter rod and lever. They get old and crack. Replaced mine, but still the car will roll forward or readward
a little when parking.

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1980 300SD Manganese Brown Metallic Clearcoat 248k
1979 300SD White 148 k
1984 300D Silver project car for daughter
1982 300D wrecked
1980 300SD 310K not running