If you don't want to mess with the wiring and plumbing, you can get a wind deflector off of an older SUV or stationwagon.
The reason that all of the dirt and grime collects on the window is that when your truck is at speed, the air going over the truck creates an area w/ a negative air pressure, so it sucks the dirt right onto the window. If you notice, older SUVs and stationwagons do not have rear window wipers, but did not need them because of the wind deflector on the back edge of the roof. this channeled air down the back window which kept the dirt and grime and spray from coming back onto it. The newer SUV's in order to look streamlined, did away with the deflector and put a wiper assembly on the rear window.
AJ