Blend door

Where can I find the blend door on our cars. I have searched the site trying to find the thread on it, but no luck. I think there was a write up about it before so if anyone has the link to it it would be great. The problem I am having is the heater air is warm coming out of the center vents, but not so much on the outters. When you stop, the floors blow a little warm and then col when you start to move again. fluid is topped off and engine temp comes up to normal, so I know it's not that. Any input would be greatly appreciated. trying to stay warm on these cold days.

stay still!!! sorry....had to.

lol!!!!!!