WARNING: This page is about a different car, the 2002 Oldsmobile Bravada, 2002 GMC Envoy XL, 2002 GMC Envoy, and 2002 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
Remove the extension panel hinge finish cover (2).
Fig 1: View Of Extension Panel Hinge Finish Cover & Retaining Screws
Courtesy of GENERAL MOTORS CORP.
Remove the screws (3) securing the extension panel (1) to the seat back.