My Suzuki Swift had very square rear windows, so I invented and implemented this Window Rain Shade.
https://docs.google.com/file/d/0B20U...it?usp=sharing
I used it pretty much every time I parked. If it was going to rain lightly, but there was going to be sun, it was great. If it was going to be sunny, it was great. It allowed excess heat to escape. You can see in this picture that I coupled it with a windshield shade for best results. Actually, for best results, I coupled it with the SedanKamm that more or less shaded the whole back window... The Window Rain Shade only took about 30 seconds to install, and less to uninstall. Roll the window down a few inches, slide the sides into place, push up so that the rain would go over it, and roll the window back up to hold things in place. It was made of coroplast that was left over from something else. It would have been nice to have a second one for the other side, that way you could get a cross-breeze. No rain ever got in while it was in use.
I should add that the Swift had manual windows, so it was easier to install. With auto windows, or an irregularly shaped window, it could be much more difficult to install.
Before I had the Window Rain Shade, or even the solar reflector, I tried one of those solar powered window fans, it was a joke, it wouldn't run unless the sun was low enough in the sky to penetrate the window, and even then, the Window Rain Shade did a better job. I also wondered how hot it would get in the car, so I got a huge thermometer and put it under the passenger seat. Most days, I would come out to the car, and if it was over 70 outside, it was maxed out at 120 (it only went to 120). One day, it was really HOT, I pulled the thermometer out and it had actually exploded and there were chunks of glass on the passenger's carpet. After that was when I invested in a meat thermometer (that you can just leave in the oven), and a solar window reflector.