You are correct. The amount of water in the air in the garage is constant and independant of temperature. Its likely to be the same in gm/ cu metre as the water content outside unless the garage is effectively sealed or has just received a wet car from washing. Thermodynamics ensures that. Damp air will migrate into or out of the garage to equalise.
However, most all chemical processes are time and temperature dependant. Rusting is a chemical process in that sense so in theory the car will rust ever so slightly faster in a warm garage than in a cold one. Maybe 3 or 4% faster for every 10C?
If you are worried about that issue, and you shouldnt be since modern coatings protect steel from rusting very effectively, then you can help by using a de-humidifier in the garage.
Family car lives outside in rain and snow and with salted roads but doesnt rust, and that cost way less than a Morgan.
