One jams the fan in the top of the window and the sun makes it go, thereby sucking out all the nasty heat, leaving the car cool.
I was never a physics major, but it would seem that if the outside temperature is say 35C (pretty hot), and the car interior is 50C say?? , the very best your going to do if the little fan completely exchanges the air every few minutes is 35C............Not cool and comfy IMHO. It's only a fan, not an air conditioner.
Or am I missing something??
Not a life changing revelation, but it would be nice to know.