If you reach under the edge of the rear quarter panel you will finda short rubber hose shaped drain. It is shaped like a heart valve if you feel up in the rubber hose with your finger. This is the drain for the area at the outer edge of the trunk. Check inside the trunk above the leak and see what you find.
Usually what happens is that heart valve gets dirty and will not allow liquid to drain from the trunk. You have to wiggle the heart valve with your finger to get it to drain.
Good luck and God bless America,