Our mail trucks get so freaking hot sometimes inside. Aluminum, no firewall or floor insulation, big windows and that damn heater core always circulating coolant. I have kinked and zip stripped the heater hose before on some of them. They actually have a valve in the heater lines that in theory bypasses the heater core when you move the control from hot to cold. The problem is they don't seem to always work on these 30 year old trucks.
