I live in a pretty dry place, relatively speaking. I have used one of those reusable ice pouches. Something like this:
IGLOO 25199 Reusable Ice Block,7-7/8x15/16x4-1/4 in. - Walmart.com
I tuck it behind my lower back against the seat. Sometimes I move it under arm or behind my neck.
The idea is to cool the blood running through my skin in that area, which of course means it circulates through my body, crudely like a cool vest.
On really hot days in my car, it helps me feel a lot more comfortable, but I have a high tolerance for heat and have never liked air conditioning much.
Your mileage may vary!