Have a look at my car this morning...
The rear center bumper, you can clearly see how the snow clings to it because
, I'm assuming, that part of the cars aero has a large low pressure zone from boundary separation...and being a hatch back, somewhat worse than your average sedan...
I was thinking, if I had a belly pan from the rear axle to the bottom of the bumper, with a NACA duct or even a scoop hanging down, to pickup air underneath the car and plumb it to a grill opening, maybe 20" wide, in the center of the bumper where the colored part meets the matte black plastic, I could cancel out some of the low pressure void and replace it with ambient pressure air...
Does a racecar like diffuser do something similar, or is that more or less to create low pressure under the rear of the car for aero grip?
Of course, if would have to be equal diameter thruout so as to not create low or high pressure inside the ducting...
(restricting the diameter would create low pressure venturi effect)
Also thinking of a modified spoiler to grab some of the clean air over the roof and direct it downward over the glass area. I remember some of the early minivans used something similar to keep the rear glass free of dirt/snow...?
As for the snow clinging to the car after the rear wheels...? I'm not entirely sure I can explain that. Maybe the air coming off the trailing edge of the wheel? Another low pressure area?