This occurs because objects that give up more energy than they take in will cool down. The car radiates into the clear night air. It does not have much area in contact with the relatively warmer ground, and even that contact is made by insulating rubber tires. This allows very little conduction of heat from the ground. The car can drop to below freezing temperature even if the environment is above freezing.
Work Step by Step
This is discussed on page 310.