[Perfectlap]

Next time find an underground parking garage that is somewhat heated. Fill two buckets from Home Depot or Lowes with hot water. (get the buckets that you can put a lid on).
Drop an ounce or two of either Optimum No Rinse or the Turtle Wax Waterless Wash (Wal-Mart) into one of the buckets. Wet down the car with a gallon of distilled water from the Supermarket to loose up the dirt/salt. Then take a chenille wash mitt and dunk it in the bucket with the waterless wash solution. Take one long pass from front of car to the rear. Dunk the sponge again to clean it off. Repeat until all of the paint is done. Grab a long handle car wash brush and get at as much of the stuff underneath and under the bumpers. This part is a bit of trick because the brushes won't hold as much water so you'll have to do a lot of dunking. Or you can buy a 2 gallon pesticide sprayer and fill it with the No-Rinse water and solution mix. Pump the handle and you can spray down all the stuff underneath and follow with the brushes. Spray again and again. I use blue shop towels to clean off the wheels with the waterless solution and to dry.
Dry the car by spraying it down with a quick detailer or a car wash spray like Griot's Garage (green liquid). Use their green or blue long-nap towels to buff the paint dry (long passes).
Sounds like a lot but I can do this in about 20 minutes (I don't clean underneath, new Porsches are not like the old). I've started using an outdoor car cover so I have to do a quick waterless wash before the cover goes on.
Putting a cover over a dirty car will only scratch up the paint.