WARNING: This page is about a different car, the 2003 Pontiac Aztek and 2003 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.
Align the rear door sill plate and push in the retainers.
Install the door sill plate by applying pressure to the tabs.
Close the lift gate.
Fig 1: Installing Procedure Of Door Sill Plate - Lift Gate (Rendezvous)