Get some PVC pipe and build a T shape out of it. Make it slightly longer than the car is wide, and make the shorter ends about a foot long, all they do is keep the main pipe from rolling. Block off shorter ends of the T and put a hose coupling on the longer end. Now take a small drill bit and drill series of holes in the top side of the main pipe, just drill through one side though, not all the way through. What you have now is essentially a sprinkler that shoots water up and is safe to drive over (thick wall PVC is plenty strong to hold up a car). Every so often you can lay it in the driveway, turn the water on, and drive over it few times to clean the underside of the car and wash away the salt. Also works good for cleaning lawnmower decks. This 'sprinkler' can also be built as a grid, not just a single T, if there is a need for it. Just keep in mind that more holes you add, less pressure you will get out of each one. In some cases that can be a good thing though as just a few tiny holes will shoot water several feet into the air, not something one desires.
If some soap is needed in the water, you can get an inline Miracle-Gro plant feeder and fill it with soap. As water passes, it will mix in with the soap and apply that mixture to the underside of the car.
If some soap is needed in the water, you can get an inline Miracle-Gro plant feeder and fill it with soap. As water passes, it will mix in with the soap and apply that mixture to the underside of the car.
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