Originally Posted by buckwheat

The red one routes any heat that has been demanded by the system to the upper vents. Accordingly, the blue routes any cooling to just the two center vents and dash side vents.

From what I can gather, it closes all vent flaps except the center two flap valves, so runs cool/heat into either depending on which one is pushed. So it should close floor and defrost vents, but leaves the rear vent alone. Note that if the system is not calling for any kind of heat, it will route air up there but no heat. Same with cool.

Why? To me it's handy when it's hot or cold, and I want cool/heat air out of those vents only - instead of the system's idead of routing air all over the car as it wants.

Interestingly, when a/c kicks in and it figures out that it's really hot inside the car, it's hot outside the car, and the a/c is producing cold air, it will do this anyway - close floor and defrost and route to center/outside.

Press the cool (blue) button next time it's really hot and you'll see what I'm talking about.

Hope this helps -