Here's a place that has one...
http://www.amerimark.com/cgi-bin/amerimark/cat_item.html?
prod=23288&media=G00797&days=XVQ
I was considering this too since I nap in the car at lunch and it's near 90F in the shade. Perhaps if 4 of them were installed that might make a difference.
I was considering hooking up a 20W flexible solar panel that would sit on the shelf in the rear window and hook it up the vent fan in the car somehow. Leaving the fan on while in the car with the windows down does keep a 10F differential according to my thermometer.