When I first brought my Mini home and washed it I couldn't believe all the dirt on the rear of my car. It was in every crevice. I was convinced the first owner lived on a dirt road. Then I drove it a few days and learned I had been sold a street sweeper.
Rain X and good wax helps a lot when its dry but in the Winter your SOL. FYI I removed my ugly rear wiper and it gets crazy dirty all the time.
Different stokes, never could understand a rear wiper delete. If wipers are so ugly why not delete the front ones? :ihih:
I have a Clubman and have the same issue. Have you ever noticed the huge motor homes that have the brushes that hang down in the rear sorta like the UPS trucks use on their rear fenders to disrupt/contain water spray that comes up. I was wondering is you used one of them between the rear wheels, right after the axle area if that would help. It might look weird but I bet it would work.
Nothing will stop the rear from getting dirty many have tried and always fail. A MINI is an Areo brick so it will always be dirty no matter what you try. The best way to keep it clean is to wash it or park it in your garage.
I don' know how some pf you can park your MINI for the winter. Mine has been in the garage for a little over a week and I am having withdrawal. So, I heading out shortly to get her dirty.