Anyone else with this problem? It's right under the driver side inner tail lamp. I imagine salt and dirt gets in there and when water hits it just keeps rusting. The rest of the paint is in great shape.

 

Mine is inside the trunk (between the outside and inside metal) right near the catcher...

I would recommend to start looking for a mint one or carbon, I went the carbon route lol eliminate the problem all together. Good luck man.

 

Replacement isn't the only option unless you're actually losing metal. You can grind the rust off and paint the bare metal again so long as you grind enough away to make sure you don't have rust under the paint anywhere.

 

I ended up using Por-15 on the spot. I already ground it down to bare metal and painted it and it came back worse than it was before. Terrible design on GMs part with this.