It is best to look for water leaks when everything is dry. First is the trunk seal damaged in anyway? Does the trunk close flush with body? Ensure the trunk seal makes good and uniform contact with the trunk (particularly important if the vehicle has been involved in an accident. You may need to pull back carpets/trunk lining and get inside the trunk with a flashlight and get a friend with a garden hose to spray water around the trunk and tail lights to determine where the water is entering. Then spray water around the rear windscreen moving around the circumference of the windscreen. Is there any rust around the rear windscreen area or body work? A repair shop will have a mechanic get in the trunk with a flash light while another person sprays water around the tail lights and windscreen to locate the source of water leaks into the trunk. Look for damage to the rear door seals, look at the sunroof if you have a sunroof the rear drains could be restricted or blocked.