I read some of this thread weeks ago, it is now 5 pages long and here at work I just don't have the time to read all the excellent posts. If this opinion was already posted then I'm sorry for repeating it, but here are my 2 cents.
Cars and trucks should fall under different rules. I own a truck and it is this that I have an opinion on. I know the reg says no stickers on the windshield or side windows adjacent to or in front of the driver. That is all fine and dandy but IMO there should be no reg about sickers on the rear window of a truck because....
1) unlike cars, trucks are not required to have rear view mirrors, just side mirrors.
2) some trucks do not even come with rear view mirrors.
3) because often trucks have box backs that completely block the view from the back window.
4) I could be wrong but I never heard it was against the law to drive a truck down the road with a load that obstructed the view from the back window (I mean come people, carrying loads is what trucks are made for)
5) I don't think cap manufacturers should be able to sell something that is against the law in most states. Now think about all the caps you see on trucks, every single one of them comes with a logo sticker on the back window (i.e. "Century" "Leer" "Ranchero" what ever it may be. If something is on the rear window of a cap (as they all come from the factory) what is the difference between that and having something on the rear window of the truck itself, it is still obstructing the view.
So IMO, based on the above, rear windows on trucks should be completely and totaly exempt from state regs about stickers on the windows.