Hello and good evening. I had to do the same thing on my 04 CTS-V. The sail panel is what that trim is called. On my car it was the driver's side. The screw that is supposed to hold that panel in place is behind that airbag emblem at the top. Pull that emblem off and you will see the recess that the screw goes in. On my car, the plastic was broken where the screw would rest against it. I 2 part epoxied a new piece of plastic over that broken section and was able to reinstall the screw. It hasn't moved since. Let me know if this makes sense and I can elaborate if necessary. I can upload a picture if needed, but go look at yours and see if my description makes sense when you are actually looking at your panel. Let me know.
Your JA Expert,