Cavemen did not have cars! Imagine how much more exercise you'd get if you walked instead of drove that car all the time!
Heheh, just kidding. My car-washing days are long gone, but I recall it could be a decent low-level workout if I really got into it. If it got your heart rate up a bit for a while, I think you can count it. I wouldn't replace my long walks with car-washing exclusively, but every little bit counts.