WARNING: This page is about a different car, the 2004 Pontiac Aztek and 2004 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: View Of Liftgate Door Sill Plate
Courtesy of GENERAL MOTORS CORP.
Align the rear door sill plate and push in the retainers.
Install the door sill plate by applying pressure to the tabs.