Yes I know what you mean - my garage often seems to me to be colder than the outside air but the more I think about it the more I realise that this is impossible unless my garage has been made into a cold room.
My garage is attached to the house on two sides. I enter it from a very warm utility room but the air temperature in the garage can be no colder and probably a degree or so warmer than the outside air temperature. . If I enter the garage from the outside when the sun is shining then it does feel noticeably colder because I get no reflected warming.
The condensation in your garage can only be caused by warm wet air meeting the cold surfaces of walls and or ceiling in the garage - could be from inside the house or residual heat from a wet motor vehicle.
The solutions you might try are: keep the warm damp air out of the garage - insulate and line the walls and ceiling of the garage -ventilate the garage a little to let out any trapped damp air - possibly consider a small, cheap dehumidifier - let the car cool down before putting it away for the night.
Peccavi, February 2009
