One more place to check:
(From the school of hard knocks)
I had a leak like that. Mine started after I had some body work done on the rear of the car. Turned out there was a hole the size of the head of a pin *under* the right rear taillight lens. Water runs down that channel (around the trunk opening) and crossed the pinhole, letting water into the trunk. I found it using a light. Try putting a light inside the trunk (at night) and closing it, and try to see where the light comes out. Also try the other way (light outside, look from inside.)
You might also be able to find some traces of where the water is coming from; take out all of the interior from the trunk, and you will possibly see white stains on the inside where the water has evaporated.
Given the volume of water you are reporting, it should not be too hard to find.