The door on the face doesn't dump heat into the cab. When the door is open it recirculates the cab air. It is connected to another door that closes the fresh air vent when the in cab door is open. Closing it draws in fresh air.

http://www.flickr.com/photos/advance-design_parts_co/8129312977/in/set-72157631868452754/

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.