WARNING: This page is about a different car, the 2006 Buick LaCrosse and 2006 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Identifying Rear Compartment Lamp & Bulb
Courtesy of GENERAL MOTORS CORP.
Install the bulb to the rear compartment lamp socket.
Install the rear compartment lamp, pressing into the lamp place until fully seated.