If it's either a selling point of the car enough that the manufacturer wants to demonstrate it accurately, or if it's something enough people care about and want to see which car suits them better, then a third party could create a simulation tool that let's you quickly go through dozens of different sun positions, weather conditions etc. and see how your personalised body shape will be positioned in each car including being able to adjust the virtual seat position, sun flaps, etc. to compare how you'll be affected by light in a different range of cars... it could be a useful feature? Hell, maybe it even extends to being able to choose the perfect custom flap size and positioning, to either get it added into a new car sale, or ordered as an after-market minor swap to improve your driving experience.