See also:

Important safety notes
WARNING Make sure absolutely no objects are obstructing the pedals' range of movement. Keep the driver's footwell clear of all obstacles. If there are any floormats or carpets in the footwel ...

Mercedes-Benz M-Class Owners Manual
...

Activating/deactivating
WARNING Any accumulation of snow and ice should be removed from the rear window before driving. Visibility could otherwise be impaired, endangering you and others. The rear window defroster ...