You'd think one of the brainiacs around here would know this off the top of their heads. As far as I can remember on, say, my Dodge truck "Max" moves a panel in the duct work that draws air either from inside or outside thru a filter. The last car I had that wasn't Climate Control...man, what was that thing... Anybody that watches "House" and believes he could function that well on years of Vicodin, lemme tell ya, it ain't so...anyway, whatever it was we had to replace a small motor that operated that same sort of door.
Someone had a complete heater/vent duct housing for sale here just recently with a pic, or a link to a pic. It might all be visible in that pic...