I know this may sound ridiculous but try a kitchen white eraser scrub pad, new product that has been out approx 1 year. My daughter who has a white Honda Accord, her bumper was apparently clipped by someone in the parking lot that appeared to either had a black rubber bumper, or a black trim piece on it, she took that eraser pad to it and it cleaned it completly off whereas she tried with soap/water and it didnt work. I on the other hand have a GMC Jimmy that somehow obtained yellow marking road paint all down the side of the vehicle, I tried everything I could, rubbing compound, bug/tar cleaner, break cleaner, even bought a $15 bottle of paint cleaner from an auto part store recommended by the salesperson and nothing- 3 years later I got so aggrevated that I tried GOO GONE- with not much effort it came off- although in your comment it appears to you have already tried that, maybe try the brake cleaner- test a spot first, I have also used that on other things on the vehicle body and it worked.