WARNING: This page is about a different car, the 2002 Buick Regal and 2002 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
Position the window all the way down.
Remove the outer belt sealing strip screws.
Fig 1: Identifying Rear Door Window Belt Outer Sealing Strip
Courtesy of GENERAL MOTORS CORP.
Use your fingertips in order to pull the sealing strip from the door flange.