Rusted Firewall

[money pit]

I bought a Barracuda a couple of weeks ago. I bought the car from a guy in So. Calif. The car was originally from Virginia. I knew the firewall had a fiberglass patch near the drivers side hinge. I was doing some work under the dash. I removed the fresh air vent and found it was rusted out. Before I do any repairs, I would like to know where the water is coming from. Does the opening for the air vent, under the fender? Can I seal this opening up? I plan on having the firewall repaired properly. Thanks

 

[Challenger RTA]

Both air vents are under the cowl. It is a two piece. Firewall and the top is the cowl. Spot welds in orange.

An other members look inside from the outside.