See also:

Mercedes-Benz M-Class Owners Manual
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Important safety notes
WARNING When leaving the vehicle, always remove the SmartKey from the ignition lock. Always take the SmartKey with you and lock the vehicle. Do not leave children unsupervised in the vehicle ...

Problems with the rear window defroster
The rear window defroster has deactivated itself prematurely or cannot be activated. Switch off any consumers that are not required, e.g. reading lamps or interior lighting. When the battery ...