Driving a long distance in hot weather without AC?!

I deliver mail and am on the street in a truck with no AC from about 9:30 to 5 every day. People always act surprised when they learn no AC in those things. They are aluminum boxes with no insulation sitting on a S-10 frame and motor that circulates hot water through the heater all day. I actually pegged and broke a cheap thermometer a few weeks ago at 135+. It is the one part of my job I really dislike. I can't wait to get in my private car and crank up the AC on the way to my AC home set at 71.
I have noticed some NASCAR underarmour type stuff with cooling tubes sewn in but I am constantly having to get in and out.. that ebay link posted with the cooling pad looks good too.