WARNING: This page is about a different car, the 2003 Buick Regal and 2003 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
Use J 38778 to loosen the carpet retainer clips.
Remove the rear side door opening carpet retainers from the vehicle.
Fig 1: Removing Rear Side Door Opening Carpet Retainers From Vehicle