True, one important thing I forgot to stress was you have to keep things out of direct sunlight. Once the items are in direct sun, then all bets are off because the radiation from the sun makes things a lot hotter than the ambient temperature, so temperatures can easily skyrocket. So usually that means storing things in a trunk, or at the very least throwing a towel over the cooler to keep the sun off. Also, one of those reflective sunshades in the windshield helps a lot, anything you can do to keep the sun out will keep your car cooler. I remember this issue used to come up on photography sites a lot, back when film was still popular.

Here's a pretty good graph showing the interior temperatures of a car over time.

http://www.randomuseless.info/318ti/temperature/interior_temps.png

Just by placing items in different areas of the car you can reduce the max temps by 20-30 degrees. The important thing to remember is all you're trying to do is buffer the temperature extremes. You will never be able to keep it at a constant temperature without active cooling, because given enough time, the temperature will always equalize. All you want to do is insulate it enough to make it through the hottest part of the day until it cools down again.

FWIW, those soft sided coolers never really worked for me, I think the zippers just couldn't get a good enough seal. I used one of those cheap styrofoam cooler that they ship frozen things in. The styrofoam was about 2" thick and it had a tight fitting lid. It seemed to insulate better than any other cooler I've ever used, plus it was free.