One more thought about a fan
I'm a fan of fans, TC. I just had a great idea for where it would cool you off best - mounted on your headrest, on the side or top, pointed at the back of your head. A little muffin fan extracted from dead electronics such as a PC power supply is 1) free, 2) 12VDC, 3) draws maybe .3A and could even be run from a 4.5 watt solar cell (WalMart auto dept, around $40, as I recall) on the back window ledge.
With it so close to your ears though, you'd have to pick a quiet one with a slow rpm to not drive you deef. Or if the headrest has an arch to it and a gap between it and the seat like some cars do, put it behind the headrest at the bottom so that the air goes between em.
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HaPPI 
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