Your car is potentially more comfortable than any tent if you're going camping, but without running the battery all night or opening a window to the elements, it's impossible to get any air flow in the cabin. Impossible, that is, without these awesome magnetic window coverings.
All you need are some rare earth magnets, some flyscreen, and duct tape. Cut the screen to fit your window, attach the magnets to the perimeter with tape, and stick it across your window frame. If you want something a little less ghetto-looking, you could probably sew the magnets into place, but the tape does provide a nice cushion so they don't scratch your paint.
To be clear, this probably won't keep every insect out, but it's certainly better than just leaving the window completely open. Check out the Instructable for more photos and a complete guide.
Magnetic Window Screens for Car Camping [Instructables]