Look under the mat in the trunk and locate the foam block that fills the bottom of the right (passenger) side trunk well. you should see a deep cavity in the foam block. take a screwdriver and poke a hole through the bottom of the cavity. The foam is only about 3/8 inch thick, and it is the lowest part of the trunk. Enlarge the hole with the screwdriver just to be sure water can seep through. If water percolates up through the hole you have a problem.