See also:

Mercedes-Benz M-Class Owners Manual
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Servicing the brakes
If the red brake warning lamp lights up in the instrument cluster and you hear a warning tone while the engine is running, the brake fluid level may be too low. Observe additional warning ...

Frequencies for the garage door opener
The radio type approval number and the frequency range for the garage door opener can be found on the Internet at: http://www.mercedes-benz.de/betriebsanleitung. ...