For service and parts needs, visit your dealer. You will receive genuine GM parts and GM-trained and supported service people.
Genuine GM parts have one of these marks:

Vehicle Care
California Proposition 65 WarningRear Side Door Window Adjustment
Warning: Refer to Glass and Sheet Metal Handling Warning in the Preface
section.
Warning: Refer to Express Window Down Warning in the Preface section.
Preliminary Procedures
Remove the rear side door trim. Refer to Rear Side Door Trim
Replacement.
Remove the water deflector. ...
Rear Bumper Impact Bar Replacement
Preliminary Procedure
Remove the rear bumper fascia. Refer to Rear Bumper Fascia Replacement.
Rear Bumper Energy Absorber Nut (Qty: 6)
Caution: Refer to Fastener Caution in the Preface section.
Tighten
40 N·m (30 lb ft)
Rear Bumper Impact Bar
Special Tools and Equipment
...
Installation Procedure
Clean and prepare the attaching surfaces for welding.
Position the roof rear header panel (1) on the vehicle.
Verify the fit of the panel.
Clamp the roof rear header panel into position.
Spot weld accordingly as noted from the original panel.
Apply the sealer ...