WARNING: This page is about a different car, the 2007 Pontiac G5 Pursuit, 2007 Pontiac G5, and 2007 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
Open the rear compartment.
Fig 1: Removing/Installing Outer Bumper From Compartment Lid
Courtesy of GENERAL MOTORS CORP.
Remove the outer bumper from the compartment lid by twisting the bumper counterclockwise.