Craig, here's another thought about this. I rebuilt both side vents on my cab but could never get them to completely seal with the vent closed. I ended up blocking off the driver side completely and left the passenger side functional. Like you, I have AC so only use the vent in moderate weather. The passenger side provides all the fresh air I need. You will probably still want to repair the rust but it does not have to be perfect if you do not plan to use the vent flapper door.
Mark
