In order to find your leak you may need to do the following: Remove everything from the trunk including any lining material and the spare tire. Now with someone in the trunk with a flashlight spray the rear of the car with water, Most common leaks are from the trunk lid weather strip and the tail lights. If no leaks are noted spray thoroughly spray the wheel wells. You should have found your leak by now. It is possible that water is getting sprayed in such a way that the leak only occurs while driving by this is very unlikely.