Mercedes-Benz 2015 GLK Operator's Manual page 288

Features
286
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 switch off: remove the connector from
X
115 V socket :.
Ensure that you do not pull on the cord.