did you check the seals on the taillights? and I am not sure if you trunk lid has drain holes that usually have rubber plugs in them that might get clogged.
flashlight....hop inside of trunk & close lid.....have a good friend hose it down and help you get back out after you look for the leaks. :-)
later, dave Reminder........ Before you criticize someone, you should walk a mile in their shoes. That way, when you criticize them, you are a mile away from them, and you have their shoes. Frieda Norris