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 Door Rubber Conduit
Courtesy of GENERAL MOTORS CORP.
Open the rear door.
Remove the rubber conduit (1) from the center pillar.