You aren't alone. I've commented many times about "dirty butt" syndrome with the Gen 1 Volts and believe me it looks just as bad, if not worse, on a black car.
The really important thing to keep in mind is that because there is so much crap back there, you really need to be careful about grit and not scratching things up. That means water. Lots and lots of water, to rinse it all away.
The basic stages are...
- A good washing with a wool mitt and lots of rinse water
- Then I actually blow off that whole area using an electric leaf blower
- Then a good going over with a clay-bar and some kind of detail spray (use lots of spray for lubrication)
- Then I rinse again
- Then I wax/polish. Personally I'm very fond of Meguiar's Ultimate Liquid Wax (be careful, there is an Ultimate POLISH that looks the same but it's not a wax).
- Generally I follow that up by puting Rain-X on that lower window part.
Maintenance in between consists of blowing things off regularly with the electric leaf blower, or using a "California duster" (lightly swishing, no pressure) sometimes.
