Temperature Control Switch
WARNING: This page is about a different car, the 2002 Isuzu Axiom. However, it is still accessible from the selected car via links, so may be relevant.
When the temperature control switch is in the "18°C (65°F)" position, the air mix door closes to block the air flow to the heater core.
When the temperature control switch is in the "32°C (90°F)" position, the air mix door opens to allow air to pass through the heater core and heat the passenger compartment.
Selecting the desired temperature will control the air flow through the heater core, allowing control of the cabin temperature.