Moisture in the Trunk of an 05 - Mine Too!
The day after I purchased it I was looking it over and noticed quite a bit of moisture in the bed. I also noticed it smelled musty. So it had to have been leaking for a while.
I took it to the dealer and they had a guy come in who deals strictly with vehicle water problems and he looked at it. He had to modify the existing seal that goes all the way around the bed. The way the corners of the seal cross over at the bottom right corner was causing a gap. Water was running down from the top and directly into the bed.
After the repair, the dealer cleaned the carpet and sprayed it with freshener.
It's been over two weeks and so far it's held up.
If the dealer gives you any grief, or if you want to do this before they give you grief, send a comment to Chevrolet and tell them the problem you are having. They called the dealer while the SSR was there to make sure they were taking care of it.
Good Luck! :seeya