Most likely, it is the cable. Mine quit last week and the replacement just got here today. I have an 83 300D and the part # is 123 542 82 07. your's might be the same but I am not positive. I got mine from a MB parts place in orlando Fl. through a mechanic there. Maybe check ebay. I paid $40. for mine and that included shipping. Looks pretty straight forward and easy to install, I'll let you know after the rain stops.

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project