You can activate the rear windshield and door mirror heating via both the center display and the button panel below the center display.
Activating via the center display
Press the fan symbol in the bottom bar 
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.Press the rear defroster symbol 
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.Activating via the button panel
Press the rear defroster button 
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.Tip
Automatic rear defroster
In climate settings, you can set the rear defroster to automatically turn on when you start the vehicle in cold conditions.
Press 
in the climate view and go to Auto rear defroster.