I have a dark-colored car and the interior gets very hot on a sunny day. However, if I put equipment in a picnic cooler and place it deep in the trunk under the back window deck, the cooler interior never gets hot enough to damage equipment and film. Also, there is a very significant temperature gradient between the floor of the car and the "upper elevations": the floor under a seat is a much better make-shift storage spot than the glove-box. I have also wrapped equipment in aluminized "space blankets" to reflect heat in situations when it is out in the sun.