I've got a leak !!!

I have a water leak under my dash, above my fuse box in my 1965 Dodge Dart 270.
Some water comes out of the vent box on the driver's side when it's raining, and I'm going down the road with it open. It appears that the leak is coming from above the vent box, attached under the dash. My Dad says there's some sort of flap that is suppose to allow the water to run past the vent box.
Is there any way to remove this vent box from the inside, in order to fix the problem ?

Thanks.