I've noticed my trunk was leaking, and other people have had the same problem. The first place it may be leaking from are the bolt holes where the spoiler mounts. Put a few sheets of newspaper in your trunk and pour water over the spoiler and where it mounts to the trunk. If it is leaking from here, you will see it drip onto the newspaper in a few minutes. If the newspaper is wet towards the back of the car, then you have the same problem I had.
Pour water right here, where it's nice and dirty
What you should see is that most of the water leaks out under the taillight, but not all of it. Some goes into the trunk, through this hole. I decided to make this FAQ after I plugged it up, my bad.
The tube is pointing to where the hole is. You'll see water leaking out of it when you pour it at the side of the trunk. I used RTV sealant to seal up the hole, caulk or any kind of silicone will work just as well.
How great are iPhone pictures?