WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: View Of Floor Air Outlet Duct
Courtesy of GENERAL MOTORS CORP.
Install the left floor air outlet duct (1) to the HVAC module (2).
Install the push pin (2) to the left floor air outlet duct.