Maybe the hatch seal is not working or is damaged. I'd check along the rubber seal for wear or damage. You didn't state the year, but the design hasn't changed much over the years. If your car is old (2012?) it might need a new seal. Another possible entry point is the seal around the taillight may have weakened over the years. One approach is to have someone in the trunk area with the hatch closed and spray water on the car to get a sense of where the entry point is. You may need to first pull out some of the carpets as well. Good luck!