I am always amazed that people run NACA ducts backwards to vent air from the inside of a car. They don't work backwards! Kind of goes with an old NASCAR saying, "If Petty won a race dragging a dead cow across the finish line, there wouldn't be a cow in the country come the next race!" Monkey see, monkey do. Take some time to see exactly what makes a NACA duct work then it becomes obvious that they don't work backwards.
Rex