There are a few iPhone car clamps that mount the phone on the AC exhaust vent. I used one like this for years. Always cool on a sunny day as the AC was constantly blowing over the phone. http://www.amazon.com/Dedicated-Swivel-Vehicle-Holder-iPhone/dp/B009GGUMDO
You asked, I answered. Reader Meghan wanted to know how she could keep her iPhone from overheating on a long road trip to Comic-Con. Here are my suggestions, and, if you have any other ideas, please share them in the comments!