Do a search on "headrests"...

...if your mechanism is similar to that of the W124 series, there is a $18 part that you can replace that will fix this issue.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car