It has to do with radiative cooling of your car. If you park it underneath a large tree it won't have frost on it.

Briefly, if your car is losing it's heat to the open air, it will cool rather quickly. The sky goes a long way up, after all, and gets might cold at night. (Assuming stratus clouds are not present trapping the heat in.)

If it is under cover, and the closest object is a large tree/shelter/etc, it will radiate its heat to that object (and vice versa). Since that object will undoubtedly be closer in temperature to the car, not as much heat transfer is going on--less chance of frost.
 

boylan, the bringer of knowledge. they should name a Gustav Holst "the Plants" movement after you. it would be full of intricacies but in the end leave you with a profound feeling of comprehension and inner peace. anyways, i think you might be right but there arnt that many trees around here... and some of the other cars are outdoors in the open too. hmm i shall ponder your advice and seek a remedy