In the hot climates, people are using shades inserted on the interior of a car to reduce the heat buildup inside the cab. They work moderately. I believe the shade needs to be on the outside of the car.
Idea. version 1. put a roller shade attached near the rear window with enough fabric to pull over the car to the front window and can anchor on the front window via a clip or to the hood.
Version2. Manufacturers can embed a shade that can mechanically be pulled up from each window via, from under the hood, under the trunk and from each side window. The shade can have a wire or plastic frame to hold the shape.
Version 3. Motorize the operation to enable a key bob to activate.
The main principle is block the sun before it hits the glass.