1965 Dodge Dart, Rain leaking in to the coupe

Stick your hands up the heater and vent box doors as high as you can go. If you reach over the edges you might can find leaves and junk up there causing water to go places it's not supposed to. It also might be wiper pivot seals or rust pin holes. There are a few restoration threads with cowl repair that will give you an idea of what you are dealing with.