Water Leak in Trunk

The simplest solution is to climb inside the trunk and have a friend spray a hose around the back end, then watch where it comes in. Spray low at first, and work upward.

My gas filler was a source of both water and gas leaking into my trunk. Gas was getting in when I would overfill it.... instead of it draining down the tube and out to the ground, it was going between the bad seal between the quarter panel and the filler body.

Rain water was getting in the same way.

Check it out.

Also, there are some rubber bung holes in the trunk. Pull your panels and carpeting, inspect all the seals by hitting them with a hose at close range from under/outside the car.