The Diagnostic procedure mentioned above is what i would recommend.

Also just to give you a heads up if you find there is motor activity and just the teeth on the extend / retract gear inside the housing is bad the fix for that is a little pricey.

Since MB does not offer a rebuild kit or even just the gear alone you have to buy the whole motor assm. which can run upwards of $300 to $400 or more depending on model.

Just a heads up.

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Automotive / Heavy equipment tech since 1997
Mercedes Benz Tech since 2002
A+, Network+ and MCSE Computer certified

With Mercedes Benz... If the part looks cheap, its more expensive then you think. If the part looks expensive... Your screwed!