Mark,
    The air for the upper vents flows through the front doors. If you open
the doors, you'll see an oval vent with a rubber seal on it on the front of
the door. This connects to an oval hole in the body when the door is closed.
On my '82 I duct tape the hole in the body to effectively shut off the air
to the overhead vents for the season. In the Spring, I remove the duct tape
and the vents are working again. Good luck.
Marve
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'82 diesel Vanagon