Water Leak in Cabin!

I'm new here, but i'll cut to the situation

My Dart has a water leak coming from somewhere in or around the dashboard. I bought the car about a year ago and removed the old carpets to put in new ones, only to find surface rust on the floor pans!!

I used a drill with metal brush attached to remove the rust from one of the floor pans, but every time it rains more water accumulates on the floor.

My friends believe the leak is coming from the windshield because it has the original rubber. I spoke to a specialist from an auto body store and he said to check the heater core and see if there is any corrosion because that is how the water is getting in.

I also question the two air vents below the dash and how they go to the vent outside (is that maybe how the water gets in?)

Any advice or experience will be much appreciated.