Upon rereading your original post, I suppose the freely spinning knob should have been the clue.....sorry about that.

If you car is configured like mine (again!) you may find it only necessary to remove the panel over the driver's legs to get to the cable and not have to remove the instrument cluster. Don't know enough about your car to be sure.

In any event, hope you get it solved easily.

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Ted
1979 240D
165,000 miles