I have never had the back of a be-Kammbacked car get as dirty as this Mirage does. This is accumulation from normal winter highway driving - not driving on any dirt or gravel roads.
Then again, I have never put side skirts on a car before this one either. Maybe they changed flow/pressure enough at the rear to make this mess.
I usually delete the rear wiper/motor assembly on my cars. I can't do it on the Mirage or I won't be able to see out the rear within a short amount of time on a wet winter highway.