There are other options that don't involve solar panels, like the new flexible solar films that some nanotechnology companies are now starting to sell... they are less efficient but A LOT cheaper, and can be placed onto any surface without affecting its aerodynamics:
http://www.nanosolar.com/
In Spain we've got LOTS of sun. When I go to work I could leave my car parked under the sun for 10-12 hours and then have enough AC to get home. Besides, the solar panel would also contribute passively to reduce the heat inside the car.