Since the moisture is on the underside, it's coming up from the floor and the rest of the air in the garage is ok. Sealing it is the best idea but a quick and easy fix is to pick up a 'blem', 'second', or remnant of roll flooring. Get a piece a little bigger than the car's footprint (7x17). Roll the car out lay it down and put the car back. Problem solved.
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