I have an 86 El Camino with 305. The heater works but the cold air comes out under the dash & out the defroster vents. After driving for a long time it will come out heater vents in dash. I changed both of the Vacuum Switches behind the face plate. Any other ideas?

Sounds like the vacuum diaphram that operates the flapper in the lower centerof the factory ducts. this part dies a long slow death & will go out long before the control switch.