Mercedes-Benz B-Class Operator's Manual page 133

If the battery voltage is too low, the rear
window defroster may switch off.
Activating/deactivating
Turn the SmartKey to position 2 in the
X
ignition lock ( page 137).
Y
Press the ¤ button.
X
The indicator lamp in the ¤ button lights
up or goes out.
Operating the climate control systems
131