Late model Vanagons (talking hardtops here) will sometimes have a black vent that is part of the rearmost side windows. Originally these had foam in them. The foams wears out and can be pulled out. There is a rubber flap that seals this rear vent unless pressure allows air to leave, then the flap can move outwards (to allow air to leave). 

My glass is removed and so are these vents. My foam is long gone (be careful handling these pieces, if you drop them you'll break a plastic corner). Next I'm having someone split these open and remove the rubber flap, and replace the flap with a screen. Might even put in a small computer type fan inside the vent.

Question is, would it be better to have the fan move air out or move air in? Or, gee, what about having one side move air in, and the other side move air out? 

Air-out would allow air heated by the engine to leave. Air-in would allow you to get fresh air if trying to take a road nap in the desert. 

I'm hoping to have some ventilation for when the vanagon is parked. My sliding glass windows (the middle side windows) are being replaced with solid glass. Though, when they were left open, they didn't seem to help that much, though opening the sliding door has always been helpful for instant relief. Right now though, with all my glass out, its nice and airy with no heat buildup. 

Thing is, with the engine in the rear and under the interior, it seems it increases heat retention when parked after driving, so maybe the vents are better positioned than the mid windows? 

I will say that heater valve sold by Terry K, was a welcome relief. For years I was driving overheated and just had no idea the heat was actually always on.