My 2001 Focus had this material in the rear, and so does my '05. Since I try hard to keep things clean, usually a shot from the foamgun, followed by a blast from the hose works pretty well. If the weather has been bad (winter salt, etc.), then the wand at the DIY carwash helps a lot.
However, on neglected cars, who knows? You detailers out there see so much more ridiculous stuff than I do, that I don't know if I have a suggestion for that.