In the mini van we used to have, my husband would start the vehicle about 5 minutes or so ahead of time, or start it then load in groceries or what have you, and turn on the AC and aim the vents at the seats...that cooled them enough in the summer to not "fry" your legs when you sat dow.
Otherwise, (to keep from sticking to the seats or sweating til you slide) I'd think an old beach towel would work, or get seat covers.
I'm short and I always use a pillow to sit on, so this hint won't work for the seat part. But slip an extra large Tshirt (sweatshirt in the winter) over the seat back. It will also keep it cleaner.
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