The key in the ignition switch is in position 1 or 2.
When the rear window heating is on, the indicator light in the switch comes on.
Attention! Rear window heating requires a lot of energy. Therefore, when the desired result is achieved, turn off this mode immediately. Automatic switching of rear window heating will occur in 20 minutes at the latest.
First, remove any snow or frost from the outside of the glass.
If many consumers are switched on or the battery is not sufficiently charged, the rear window heating may switch off (in this case the control light in the switch flickers) and turns on automatically when the required voltage appears.
[Material borrowed from an online resource MercedesMan.ru]