Mercedes-Benz 2015 M-Class Operator's Manual page 353

Using the 115 V power socket
To switch on: switch the ignition on.
X
Open flap =.
X
Insert the plug of the electronic device into
X
115 V power socket :.
Indicator lamp ; lights up.
To turn off: disconnect the plug from 115 V
X
power socket :.
Ensure that you do not pull on the cord.
Features
351
Z