why does the dust not get sucked into the forward facing vents on the roof
scott
It does not actually. They are collecting clean air unless:-
a. you are going very slow with a tail wind.
b. your vehicle creates a dusty airflow over the roof.
c. you are following another vehicle too close.
d. you pass a vehicle coming from the opposite direction that has kicked up a lot of dust.
They are like the vent system on your car.
If you set it for outside air you get clean air (ignoring exhaust and invisible odours) but if you leave it like that and drive in the above situations you will get dust in the car.
Anyway that's whats my testing proved.