I had the same problem and it was the trunk vents. I think you can see them behind the trunk carpet lining on the sides of the trunk.
To repair the leak I recommend that you remove the rear bumper which is really not very difficult- just a few bolts here and there, maybe a screw or two in the wheel wells, and the thing pops right off. Once you do that you will have easy access to the trunk vents on both sides. Pop the vents out, remove the rubber/foam seal and replace with gobs of black RTV sealer.
Let dry and your problem should be solved. Just remember to put the bumper back on.
