I currently live in the Phoenix, Arizona area, which gets mighty hot in the summer. This summer, we've had several days around or above 110 degrees. I have twin baby boys, and despite cracking the windows, using reflective seat covers and running the A/C full blast when driving them around, their backs are just soaked with sweat when we reach our destination. The seats bake in the car in the sun for hours, then you put a baby in it and they never really cool off. So I devised this method for cooling their car seats with pump-recirculated cold water. The end product is a cooling pad to fit underneath the car seat cover behind their backs. This can be easily modified to fit adults too.
When I was about 75% done with my system, I stumbled upon this
instructable. It's pretty similar to my setup, and I ended up using his source for some of my tubing connectors, www.usplastic.com, and his idea for the quick release connectors.