Its all Aerodynamics, the air flows under the car then Eddys back around the back of the car and up. Did you ever notice in the winter your rear window is always covered with crap?. Most suv's and mini vans have this happen. If avail for your car, get a rear window deflector. It mounts above the rear door and forces wind down across the window when your driving. Aslo mud flaps will help but not solve your problem. good luck.