Front Defrost
WARNING: This page is about a different car, the 2002 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.
The front defrost mode is the only setting that is not controlled by the automatic HVAC system. When defrost is selected, the A/C compressor is turned on. The blower motor will run and air will be directed toward the windshield, regardless of coolant temperature. The rear window defogger has no effect on the HVAC system.