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.
Open the ashtray.
Fig 1: Identifying Rear Floor Console Ashtray
Courtesy of GENERAL MOTORS CORP.
Depress the retainer at the top middle of the ashtray while pulling the ashtray rearward.