As someone who frequently takes roadtrips that mostly involve camping and/or sleeping in the car itself, I can certainly appreciate this little invention–so simple that I can't believe I didn't think of it (tho I'm not thinking of putting this old camper water tank I've got laying around to some good use now…) The road shower is basically a water tank that you mount to your roof rack, you then pressurize the tank with any gas station or at home air compressor to feed water through the attached hose and shower head (or garden hose spray nozzle?) –and voila! You've got yourself a mobile shower for the road, or a mobile hose for cleaning dirt and grime off whatever equipment you use for your adventures. The water is heated via solar, tho I'm not sure how well that would work in colder temperatures….
All this can be yours for around $230! Head on over to their website and check it out.