i would just like to say that i am REALLY impressed with the answer from Mr. todras ! i had that same leaking trunk problem, and the dealer ordered a new trunk seal, without ever even looking at my car. needless to say, it didnt fix it.
i ended up getting pissed off because it was getting my speaker box wet, and 3/4" MDF board doesnt last long after being wet. also, in the winter, the water would FREEZE the box to the floor, and anything else in the trunk. anyways, i fixed it myself.
first, i removed all the interior 'carpeting' or 'covering' on the inside of the trunk. then, i had my uncle spray the back of the car while i climbed in the trunk through the fold down rear seats.
it was easy form that point. i found the leaking vents, and fixed it!
i actually removed the rear bumper when i did it, but i did that for more than just the leaking vents. i had gotten rear-ended, and there was a long horizontal crack on my bumper cover. i wanted to take it apart to see how it works!

just so you all know, the vent fix was very easy once i had the bumper removed, AND it was very easy to remove the bumper. just nuts and bolts friends... piece of cake.
i did not use new gaskets on mine, i just cleaned everything really well, and used a "liberal amount" of silicon. it was actually fun!