my father has a 2003 malibu and every time it rains trunk is soaked
today is the fourth time it is back at stealership. i tol him if they dont fix it this time he should call a lemon law lawyer, they have replaced rear window,all seals on lid and re undercoated car twice

Usually, the water flows down the window and along the inside "lip" between the trunk opening and the body. Is there something deflecting this? Might want to open trunk and run a water hose over the roof of the car to see where the water comes from. Of course, if it is a 2003, then you will have to fight GM. Isn't it a shame that you have to fight the dealer while still under warranty?

Does it get wet sitting outside on a rainy day or only when driven on a rainy day? This will tell you whether the water is dripping down or being thrown up from wheel splash. I've heard of leaks caused by missing unibody welds.