How can I keep my iPhone / iPods cooler in a hot vehicle?
Place the electronics that contain a lithium based battery in the shade and keep windows open a crack. If possible, place the vehicle in the shade or use light or reflective colored windshield shades to reduce the heat that stays in the car.

Usually the trunk (or a well insulated bag) will lag the passenger compartment many tens of degrees and warm up several hours slower than something in the main cabin.