Trunk leaking. Why???
Because water is getting in where it should not . You have seal cracked or the rear window seal has failed. Sorry but this is a problem that can only be solved in person . You can use a garden hose and run water on the car and you might find it.
Probably either the trunk gasket needs to be replaced or there’s a leak around the third brake light. You’re not the first to notice it. You can find the same complaint mentioned on Malibu forums.