I’m speculating that the roof is thin metal or fibreglass, and/or has metal frames. These act like cold-pipes to the outside world. During the day the air in the garage is warmer and can hold more moisture, at night the warmest moistest air rises and contacts the icy cold surface and bingo all the water transfers to that. This is repeated until all the moisture in the air is on the chilly inside surface where it can then rain from.
If it has a pitched roof, something with some slope to it, I would consider making up an inner tent of tyvek (a very tough fabric used as the waterproof breathable layer in house roofs walls etc), and suspend it close to the inside of the roof to allow the drips drain to the walls away from the car. Maybe even some little gutters? Tyvek is not cheap but it is superb value for money.
Our conservatory roof has ali beams, and despite these being covered outside and inside the inner plastic covers still drip water if there is the least hint of moisture in the conservatory on a chilly night. In the previous version the old wooden beams never had a hint of condensation.
My garage door has a fibreglass skin and the inside of this used to run with water in the winter, mainly because the warm air in the garage held moisture from the warm house around it. At least the cold door was wet and not the warmer car. However I then lined the door with a inch thick layer of celotex (foam sheet with ali facing on both sides, BUT also a potential fire hazard if left near something that can get it started), and sealed this all around to the structural ribs of the door apart from a few drain/breather holes at the bottom edge. Now no drips partly because the breathers only see low humidity cold air from near the floor and outside, and mainly because the new inner surface is is not cold. This also makes a big difference to the temperature in the garage, and the fan heater is no longer needed.