To better help you; take it to the GM dealership (or any for that matter) and they will replace the tail light under warranty. GM will replace anything (except wiper blades) for 3 years or 36,000 miles bumper to bumper.
You probably need to remove the carpeting and side mats in the trunk before you can access the fasteners for the tail light housings. Most sedans that I have had (GM and Ford both) have the tail light fasteners hidden my that stuff.