A dilemma if ever there was one.

So get yourself a carwash tent. A series of poles with wide feet that you position at the four corners of your car. A connector bar that goes around the top has downward-facing nozzles at every-few-inch-intervals. Two bars that slide underneath your car have wide-spray nozzles to get the grit underneath.

Surrounding the whole thing is a simple tarp. Because cars are of varying sizes, the tarp is designed to come in four pieces that will overlap almost regardless of car size.

The device will have an apparatus containing pre-rinse, soft soap, and clear coat. Powered by a water wheel, the apparatus will have a flow meter that automatically switches from one additive (or nothing, for the rinse) to another after a certain amount of water has passed through. You can shut it off to hand-clean stubborn spots, turn the thing on again, and it'll pick up right where it left off.

Hook this up to your garden hose, open the valve on your house, and walk away for 15 minutes.

Come back, and your car is clean, with no muss and no fuss.

EDIT: Available in larger, taller versions for SUVs, cement mixers, and farm equipment. Not for use on dogs or babies.