I live in Henderson, NV. We're right outside of Las Vegas. It was 97 degrees here today, and I'm betting this puts us on track for another summer chock full of days above 110F. Considering that my son was born in early August and it doesn't really start to cool off around here until Halloween, I'd call this his third summer. Ideally, we'd like to keep him rear facing until 5, but he keeps growing and growing. Which wouldn't be too bad if all of his height weren't in his torso and head. (he's starting to outgrow 2T shirts, but he still wears 18M pants)
I drive a Corolla, and my husband drives a Cobalt. Neither car has rear A/C. I agree with the previous posters on aiming your center vents at the ceiling. It really does make a large difference in the temperature of the back seat, and I know this to be true because I've only recently stopped riding in the backseat next to my son. In addition to the tips that have been given, I'd also suggest keeping the windows cracked when the car is parked as much as you can.
We also keep an ice chest with blue ice packs in it and put the ice packs in his carseat while we are running errands. We also have some teething rings that are designed to be frozen (they have a gel in them instead of water. We've found them at Target and Babies R Us) that we toss in the ice chest as well. This year we're also going to start freezing bottles of water to take with.
As far as clothing goes, if his outfit has knit shorts we usually leave them on when he's in the car. If the shorts are denim, we wait until we reach our destination to put them on. (this assumes he's wearing a t shirt and not a onesie. If he's wearing a onesie, I've been known to skip the shorts completely if it's hot enough.) This year he has sandals that he likes. If he didn't, I'd leave him barefoot in the car. Another trick I've read about is to dampen or wet their hair, so sometimes I will plan his bath to be right before an outing.
Anyway, I hope this helps you keep your little one a bit more comfortable in the heat. Summer is a swear word around here.