Water in trunk after heavy rain

Hi. i have a 1974 Dart that collects water in the trunk after a good rain. I have been just toweling it dry when this happens. I tried searching trunk leaks and water leak with no luck. Where is the water most likely entering? All i can guess is the rear window seal and if that's true, what product would you recommend? I'm guessing this is a big problem with Darts, as I've heard that a few have rusted out trunks. Thanks!