Turnip, I recall that this button controls the fan unit located over the N/S rear wheel arch i.e. for the third row of seats. This unit can be heated, independently of the front and contolled by the dial over the N/S centre pillar, as it has a seperate matrix and coolant feed. However as it does not have air feed from the air con unit in the engine compartment then air blown from it cannot be cooled. You can however get an 'air con cooled' air feed to the second row of seats via the floor level central air feed which is controlled by the front fans with air distribution control pointed to feet. If you're not getting cold air with your air con switched on in warm outside temperatures then the air con circuit may need to be re-gassed. Do a search as this topic has been covered before and also comparative tests, I recall, between outside temperatures and temps at the vents. Hope this helps, Jeff.