In full sun a car can easily have 1+kw of heat gain. Combating that with AC takes a lot of energy especially when you consider how long a car may be sitting out in the sun. But, cutting down on heat gain either by parking in the shade or having reflective shades can get you to close to air temperature fairly easily. However, combining would make things a lot easer.

So, if your willing to go all out and spend a few thousand on modding your car. Automatic shades for every window, a fair sized solar array on the roof and an independent AC system designed for continuous operation.