2008 Mustang GT, with the AC off and cool weather outside, I still get warm air blowing from the vents. I notice it the most after the car sits for a little while, so I just figured it absorbed heat from the engine but even if I drive on the highway it won't really cool off the air. How is the temperature of the air controlled? Can the heater core be bypassed or valves put on the heater hoses to keep the AC colder in the summer?