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

If you're more than a couple miles inland you've got nothing to worry about. If you are within a few miles of the ocean it might not be a terrible idea because of the sea air and coastal layer. As far as time of year it's not really relevant, if you've been in Carlsbad more than a year or two then you know that "June gloom" with the coastal layer would be worse for humidity than January or February usually is.

My car lives outside year round and we get twice as much rain here as Carlsbad does and a decent amount of fog in the winter too. If it has good paint you shouldn't have a problem. It's a car, not a sugar cube.