FWIW...I did ask our regular UPS driver about this (AC in the trucks) earlier this week. Apparently some of the new trucks have AC in the cab, but not surprisingly, with the doors open all the time it is of little value.
He did say that drivers in the hotter areas of the country consider this subject a big deal though. That makes sense to me...I can only imagine how hot it would get in one of those big brown toasters in say southern Arizona.
Not so much of an issue for the midwest drivers though.