2008 GMC Acadia Owner's Manual - Page 171

AUX (Auxiliary): Press this button to turn the rear heating and air conditioning on. See Rear Air Conditioning and Heating System on page 3-36 or Rear Air Conditioning and Heating System and Electronic Climate Controls on page 3-38. Dual Automatic Climate Control System The heating, cooling, and ventilation in your vehicle can be controlled with this system. Your vehicle also has a flow-through ventilation system described later in this section. Rear Window Defogger The rear window defogger uses a warming grid to remove fog from the rear window.