Insulation is used to reduce the flow of heat (or electricity/sound/...). Insulated walls/roof/floor help keep the heat in a heated house during the winter, and help keep the heat out of an air-conditioned house in the summer.
By using enough non-sleeping comforters/blankets/sleeping-bags to insulate the top/bottom/sides of a cooler, one can keep a cooler cool (keep the heat from warming it up) even in an enclosed vehicle sitting in the sun. Eventually more ice/dry-ice will be needed, but one can greatly extend the life of the ice this way.
For the best results, use a great cooler, multiple layers of non-sleeping comforters/blankets/sleeping-bags, and keep as much heat out of your vehicle as possible. Park in the shade if possible, use reflective car window sunshades, and consider creating a "false roof" to shade your roof from the sun.