replace the weather strips and gaskets!, make sure the door is aligned correctly. If this dont solve the problem. Visually inspect the inside to see where its actually leaking from, by getting inside the car, while a friend uses a waterhose to pour along the sides of car and trunk. then, Figure how to seal it from therel. If you have to, use gasket RTV sealant (black gooey stuff) to repair the damaged weather sealants.