The air conditioning button in the tabin the climate view.
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.
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.