I have this scrubby thing that I use to get bugs off the front of the car and it gets the rubber off the back just as easy (use LOTS of water while doing it and rinse it well after use)...I picked it up at Sams Club.
I do not get it wet I wipe it off with my hand but do not rub to hard then I wash it off with soap and water and next time make sure to have it waxed good then it will come off with little trouble.