If the temperatures were to stay consistent throughout the day, then even with the best insulation the temperature of the item will eventually reach equilibrium without any extermal source of heating or cooling.

However, since the temperatures vary, what the insulation can do is mediate the temperature extremes. So if the peak temperature was 100 degrees, and the low was 50 (and assuming a balanced temperature graph on each side), a well insulated item would average around 75 degrees (+/- some amount), depending on the insulation and where in the car it is stored. Most likely you would see a shift in the temperature peak where the peak temperature of the item could be well into the cooling cycle of the ambient temperature. If youv'e ever opened a car door after a hot day and wonder why the inside is so much warmer than outside temperature, it's because the car itself is acting as an insulator.

Storing item in a styrofoam cooler or insulated bag is a cheap way to protect any sensitive items in a car. When I used to carry camera equipment in the car that's what I did. Even during the hottest day where the interior of the car would be too hot to touch, the items in the cooler were still comfortably warm. Not a very scientific test, but good enough that you could tell right away it was effective.