Thank You to all who replied. The answers confirmed my thought that as far as I am concerned, on my truck, the passenger side cowl vent is useless.
The instructions for the AC/Heat unit I am going to install says to close off the vent and recommends putting insulation in the cavity between the outer and inner sheet metal.
My thought was to weld and seal a piece of sheet metal on the inside of the louvers, leaving them on the outside for a stock appearance.
My wife, on the other hand, says I should cut out the louvers and weld in a smooth piece of sheet metal. She says, "there are no louvers on the driver's side".
I am going to weld the upper cowl vent shut and seal it because over the years it has been a source of water under the dash and down the inside of the firewall. My thinking on closing off the louvers (either method mentioned above) is the same, seal off potential sources of water.
Thoughts?
