Open the climate view in the centre display and select the tab for Rear climate.
Press 3rd row climate.
The air conditioning is activated/deactivated and the button illuminates/extinguishes.
Note
The air conditioning for the third seat row is activated automatically if anyone in the third seat row puts on a seatbelt.
It is not possible to manually activate the third seat row's air conditioning if the main air conditioning is deactivated or the second seat row's climate control* is deactivated.