not a bumper sticker but I have seen jeeps that have windshield stickers up high. Some are upside down and say "if you can read this, roll me over" a couple say "hummer recovery vehicle" I also have a liscense plate frame my mom gave me way back that reads "in case of rapture, this car will be unmanned"
I saw a 1974 Ford LTD, rusted all over, taillight broken out, vinyl roof torn and flapping in the breeze. The cleanest thing on the whole car was a bumper sticker that said, "My other car is a piece of crap".