I finally figured mine out, and there is no leak that is not designed into the trunk lid.
When the trunk is closed, the seal is too good. When it rains and the temperature drops several decades of degrees, it creates a vacuum inside the trunk. Now those little rubber plugs at the four corners of the trunk lid are sitting in rain water. They have holes in them. Water is pulled past the rubber plug'* contact with the body through those little holes and into the trunk lid itself, where it can then collect in the pocket at the bottom. When you open the trunk lid, the water spills into the trunk.
Tip: Open the trunk lid just a little bit, and let the water drain out first.
How do I know this? My trunk lid never has any water in it, no matter how hard the rain is, unless the temperature also drops a large amount at the same time.