Sounds like a silly question, but I have rust through my firewall from a blocked cowl drain and the pain in the butt issue of repairing the damage on such a hard to reach area got me to thinking...
The cowl vent/drain area is only there to allow access to the wiper system, correct? Why not make a removable, sealable panel that caps the cowl drain. It is removable for wiper system service and sealed to prevent water access. Too simple? Am I missing something?

 

IIRC, it's part of the flow through ventilation system, along with the body pillar exhaust valve (vent at the door).

EDIT. The following is from my owner's manual: "With the side windows closed, the flow through ventilation system provides outside air flow into the front air inlet grilles, though the vehicle and out the rear air exhaust valves."

JamesC

 

But how does it get from the cowl drain through the vehicle? And if the cowl drains are clear, you have an open system anyway...

 

The AC system/blower motor draws air in through the cowl vent, blows it in through your dash vents, then those little black plastic vents in your door jamb, just above the striker, let the air pressure from inside out. Without the door jam vents, the air pressure inside would build up until the blower motor was strong enough to pull air in through the cowl vent.

In other words? Don't seal it shut.

 

Here's a bit more from the manual:

Your vehicle has a flow through ventilation system that provides a supply of outside "ram" air into the vehicle when it is moving. When the vehicle is not in motion you can get a steady flow of outside air into the car with the heater or air conditioning blower running.

JamesC

 

Does the cowl mater if you no longer have a heating or cooling system?

 

I would think if the cowl is open at the top and water drains out the bottom behind the fenders, blocking the top would not cut off airflow, since if it is part of a flow through air system, it would just draw air through the drains at the bottom...

 

Not likely very much since the drains are not in a high pressure area (like the cowl).

 

you are missing a factory assembly manual

 

What do you mean Dave?