I recently solved my car's odor problem so I wanted to post my story, hoping I could help others who might have the same problem. A couple of years ago my car started getting a musty / moldy smell, kind of like a damp basement. I searched numerous times throughout the car for signs of leaks and moisture with no success. I complained to my dealer and they assured me they knew the cause. They felt it was the A/C system and they performed a system deodorizing service. Unfortunately, this didn't help at all. About a year ago I noticed a very small amount of condensation in the spare tire bay along with a mild musty smell. I figured this came from a couple large pieces of frozen snow that I remembered leaving in the trunk last winter, so I immediately air dried everything and reassembled the tire, tools and insulation. Unfortunately, the smell persisted.
A couple of months ago, my buddy and I were polishing and waxing my car, he noticed that the top front-end of the drivers side tail light had a small amount of play in it. You could press down on it and it moved just a small amount. The other tail light did not. After pulling the tail light assembly we found that the seal that attaches to the body and receives a small nub from the tail light assembly, was missing, allowing rain water to enter the body. I immediately remembered that the dealer had replaced a turn signal bulb (under warranty) around the time I started noticing the smell and something must have happened to the seal.
Anyway, it's a simple and inexpensive fix. I'm including a scan of the GM parts diagram, highlighting the part and it's location on the body. The seal is number 16 on the diagram. The GM part number on my receipt is 13320837 but the description is Retainer. I'm a little confused by this but I'm sure your parts guy should be able to navigate you to the right part. Please let me know if you have any questions and I hope this simple fix is the answer to your car's odor problem.