To be honest it does not sound like ventilation is the issue here.
To understand why condensation occurs in any given scenario requires an understanding of the relationship between wet and dry bulb temperatures, relative humidity and dew point. This relationship can be ascertained from something called a Psychometric chart. You can see one on this site by clicking on the appropriate link. See
here
A given volume of air can hold a maximum quantity of water vapour at a fixed temperature. However raising the temperature of the air allows it to hold more water vapour, whilst cooling the air causes the vapour to condense.
In a house large volumes of water vapour are produced by cooking, bathing and the occupants breathing. This moisture needs to be expelled from the building by ventilation. Rising warm air takes much of this vapour into the roof space, where the air is cooled and condensation is likely to occur. Hence the importance of good ventilation in the roof space of a house.
However, it is unlikely, unless you are spending a lot of time in there, that much moisture is produced in your garage. In all likelyhood the air in your garage is generally at equlibrium with the outside air, so ventilation would have little or no effect.
I cannot be certain (these problems can be awfully difficult to diagnose), but it seems to me that your problem is likely to be caused by a phenomenon known as "Clear Night Radiation".
It isn't something I am going to find easy to explain unless your Physics is up to scratch...... Heat flows from a hot body to a cooler body by radiation and the greater the difference in temparature the faster the transfer of heat. The coldest thing in the universe is space itself, so on a clear night (no cloud cover) your relatively warm garage gives up heat at a massive rate of knots to the blackness of the sky.
This is the same phenomenon that causes your car windows to ice up (even when there is no ice or snow elsewhere and often when the ambient temparature is actually above freezing.
Clear Night Radiation is actually a very big problem with some types of building, notably aircraft hangers where it can actually rain indoors because the roof areas are so great.
If I am right then your only practical solution is to provide some heat (it wouldn't need to be a lot). The heat will have two physical effects. Firstly it will raise the air temparature, allowing the air to hold more moisture. Secondly it will increase the air pressure in the garage which will tend to ensure that air flows out of it.