D3 Handbook wrote:

Defrost mode
If the windscreen is misting or covered in ice, press button to activate the automatic defrost programme; the system will immediately direct its output to achieve maximum screen clearing by:
• setting the blower speed to an appropriate level.
• distributing air flow to the screen only.
• deactivating air recirculation (in certain circumstances).
In addition, the rear and front* screen heaters will be switched on (or their timed operating cycle will recommence if they are already switched on).
Press the button a second time (or select ’AUTO’ or any air distribution control) to leave the Defrost mode. Heated screens will remain on until their time-out period has expired.