I saw an idea similar to this about a year ago, but am too lazy to pull it up. At any rate, my model has a few added design motifs.
Struts on both sides of the car, at the car's midpoint. semicircle connects the two struts and hangs over the car, is made of 1'' PVC or something similar, with holes
drilled on the inside of the arch, so that when water is supplied, it shoots down at the car. The arch sways via semicircle runners bolted to the struts. the runners pull the arch to one side and back again, over and over.
One strut is connected to the soap box. The soap box is essentially an extraction chamber. the water enters the soap box where it flows through tubing made entirely out of solid soap, dissolving some of the soap along the way and taking it to the car. as it continues to flow the soap tubing slowly dissolves, until the box is just an open chamber, ready to be filled with another soap tubing slide.
So, connect garden hose to soap box, box to strut, and turn on the hose. the first ten minutes will be a soapy wash until the soap is gone and the rinse commences. Come back in 20 mins and your car is so fresh so clean.
Perhaps have an optional compressor attachment for a more vigorous wash.