I dunno if anyone has seen these. I just stumbled upon one whilst searching for solar stuff on craigslist. I have no idea if its effective at all, but its a neat idea.
Looks like there are at least two kinds out there. Same idea each. Solar panel powers a squirrel cage fan that blow air out of the car from a cracked window. No ratings for max CFM or anything of course...
Auto Vent II TO230 [Auto Vent II TO230] - $39.95 : NewDavincis.com, Great Inventions and Products for sale
Solar Auto Vent
And if you're in the Milwaukee area:
Auto Vent II, Solar powered fan vents air from car