Depending upon window size, glass tint, altitude, and other factors, parts of the interior may reach temperatures in excess of 200 degrees. Once this occurs, gravity goes to work, pulling these items to the floor. Even if your car's interior were immune to extreme temperatures, the items in it may not be. Cameras and film are sensitive to direct sunlight and hot spots. An audio cassette can warp at 198 degrees while an aerosol can may explode at just 120 degrees.
Several solutions can alleviate heat build-up.
Some are as simple as parking in a shady spot or cracking a window. In this age of automobile theft, another option is a sun visor that covers your windshield. More expensive wrap-around visors are also available. These cover the windshield as well as the windows. Professional window tinting is a good idea, but be aware that many states have laws governing this practice.