This is found inside your dash.  If you have an L-body it's located just behind your chrome switch-head that control your A/C vents, blower speed control head thingy.
 
If you have a Shelby/Dodge car...there's about a 99% chance that at some point over the past 35+years you've seen one of these go bad...and when they do, you often will only get air out of one location (e.g.  only out of defrost, or only out of vents, etc..)    I can't even find the part number for this thing, but I know they are dang near impossible to find, especially in mint condition like this one.  I'll probably regret selling it.  This one feels nice and soft (not rock hard) and is in excellent condition.  We only have this one available.