WARNING: This page is about a different car, the 2003 GMC Sonoma, 2003 GMC Jimmy, 2003 Chevrolet S10 Pickup, and 2003 Chevrolet Blazer. However, it is still accessible from the selected car via links, so may be relevant.
Push the weatherstrip onto the pinch weld flange, beginning from the top corner of the endgate opening.
Completely seat the weatherstrip around the endgate or the liftgate opening.