[snowblind]

ANY holes/cracks in your floor, doors, etc will let in fumes and having anything open (other than thee kick panel vents) will make it worse. Opening the rear hatch will make it MUCH worse.

I cut out out a bunch of the rear floors in my Jimmy and drove it a few days will plastic sheeting laid down. With all the windows closed the plastic lay flat but as soon as I cracked the window or even the vent window the plastic floated up two feet off the floor. The air outside the truck is rushing by so fast that when you crack a window it sucks the interior air out and replaces it with the turbulent air underneath and behind the vehicle.