Tony, You could put a humidistat in the garage and check the level. If it is often over 50% it might be worth getting a dehumidifier. They aren't cheap to run as it is basically an air conditioner. I run a Danby brand in the basement and it is very effective. If the concern/issue is mostly the interior of the car you could use desiccant to absorb the moisture.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer