I've got the same problem with my 2005. I just bought it used and the dealership has been working with me on the issue. My trunk lid had actually seeped water out into the latch and rusted the portion of the body where the latch hooks.
They replaced the bezel and fixed the damage but it still leaks.
Water pours out of the trunk lid through the wiring conduit when the lid is opened.
One of the things they tried was adjusting the rubber feet on the bottom of the lid that keep the lid sitting level on the bumper. They said that they had problems with water working its way into those openings when the car was driving in heavy weather.
This slowed down the leak but didn't fix it. The trunk lid fills up with water during long rains when it's parked. When they power washed it, the couldn't make it leak. The trunk doesn't seem wet on its own but the woofer in the back deck has died in the last few weeks. My CTS doesn't have a sunroof so no sunroof drains to worry about.
ewill3rd I'm going to recheck the drains myself just in case the dealer didn't. Thanks!
Savy let me know what you find to resolve the problem and I'll do the same for you.