[dsraven]

I suppose you could tie or tape some ribbon to the sheet metal or window of the area where you may wanna install a vent, then drive the truck and see what the ribbons do. kind like a cheap mans wind tunnel.
good point on the noise thing though and possibly the exhaust fumes, depending on the aerodynamics of your truck. I think, with the little flap door things, they are usually behind some sort of upholstery. not too many newer vehicles have a bare cab wall. that may make a difference for sound. the upholstery would need to be vented to the cab area from the backside though, for access to the flap door things.