WARNING: This page is about a different car, the 2008 Cadillac SRX. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: View Of Cargo Lamp Bulb
Courtesy of GENERAL MOTORS CORP.
Install the bulb (1)
Locate the rear dome lamp assembly onto the headliner.
Push the rear dome lamp assembly in order the lock the retainers.