Hello, the water is probably entering thru the rear trunk area. here is some info on this concern. i am sure your dealer can correct this concern under warranty. i hope this info was helpful!!
Water leak into the trunk or rear floor of the passenger compartment.
Remove the Rear Compartment Sill Trim Plate . Inspect for water entering the trunk through holes in the Body Rear End Panel (1). If water is entering the vehicle, remove the Rear Bumper Fascia . Remove any remnants of the Seal Patch (2) that covers this hole and replace it with p/n 15820928 or equivalent.