Just before I left New Jersey on September 14th, I camped out for two weeks at the golf course I worked at. A late summer heat wave rolled through during those weeks. Some things I learned:
Park in the shade, all day, every day. At least park in the shade during the afternoon. A day of sun can keep the interior of your vehicle 10+ degrees over the outside temperature ALL NIGHT LONG. If there is no shade, buy a giant sheet of reflective plastic and stake it down over your entire vehicle. If that is impractical, use insulating covers on all windows. If that doesn't work, sleep in a tent or under the stars.
In the evening, relax, read a book or something, sit on your butt in a cool creek. When ready for bed, blast that A/C, and cool down the interior 15 minutes before sleeping. If your windows get covered with condensation, you know you are on the right track. Wipe your skin with a wet cloth while the A/C is running, clean all that sweat off and cool down even further. Avoid even the mildest exertion. When you turn off the ignition, roll down the windows immediately (hopefully bug screens are already applied), turn on your 12 volt computer fan, and lie as still as possible in a comfortable position. Think about snow if it helps. As your car interior gradually warms and gets more humid over the next half hour, your body should adapt as well. Avoid tossing and turning, you will just get more uncomfortable. If possible, park where there is a breeze, no matter how slight. Sleep on a hard camping pad, NOT on a thick warm memory foam mattress topper. Use a threadbare cotton sheet as a blanket.
If the dewpoint is over 80 degrees, you will be miserable in spite of everything you try. Get creative. Set up camp in a cool culvert among the mice. Sleep with your feet in a creek. Learn to sleep sitting up in a 24 hour restaurant. Sleep in the root cellar of an abandoned house. Fill a waterproof bag with ice and use it as a pillow. Sleep in your gym's air conditioned locker room. Learn to live without sleep. Beg gas money from strangers and high tail it for higher elevations. Or just adapt, as most of us will after a few weeks of misery.