Do you keep a dehumidifier in your garage to keep your classic car from turning into a pile of rust?

Dehumidifiers and desiccants are only effective in spaces that are relatively well sealed up and have few air changes. Most garages with leaky doors likely do not meet that definition. A car with the windows rolled up would probably respond well to interior humidity reduction.

Humidity monitors are pretty cheap. If you are considering spending much money doing this, you should probably buy one and track the humidity to see if you are really accomplishing anything.