You're right - they are built to be outside, but not in a humid garage. Overall humidity is a given, I'm talking about the greatly increased amount of humidity caused by fish tanks. It can't be good for metal, or wiring, or cloth/upholstery, etc. Outside, the cars can dry out, as the weather changes. Being bathed in humidity all the time... I can only imagine is harmful for cars. No ?
Unless he isn't driving the car at all I don't see it being an issue, storing a car that's currently not on the road in there w/ the windows open wouldn't be too bright but if he is using the car I don't sthink it'll hurt. Most cars are out of the garage the majority of the time anyway, plus garages tend to be more aerated then houses, drafts from the gragae door and the windows would also help.
And yeah they usually have vents too, mine does. For when you drive a wet car in ya know
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