WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
Ensure that all 3 retaining tabs are attached to the sill plate.
Fig 1: Rear Door Sill Plate To Door Frame (TrailBlazer EXT & Envoy XL)
Courtesy of GENERAL MOTORS CORP.
Align the retaining tabs to the door frame slots.
Push down on the sill plate, seating the retaining tabs to the door frame.