OMG, this is ironic, I JUST fixed this problem. As prototype5 said, the problem is right by the lights. If anyone has replaced the lightbulbs, they may have broken a plastic guide piece on the back of the tail light housing.
Also, check on the paint line right by the tail lights- it's where they connected the two metal parts of the frame. On my car, right by the rubber tunk seal, where those metal parts connected, there was a small 1 inch crack in the paint. The small crack was on both the right and left side of the rubber trunk seal. After sealing those up I've had no leakage problems.