WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: View Of Rear Shoulder Belt Guide Assembly
Courtesy of GENERAL MOTORS CORP.
Push back the rear seat back.
Align the rear shoulder belt guide to the lower rear quarter trim panel.
Install the push-pin retainer to the rear shoulder belt guide.