sounds like you may have a vaccum leak or a broken line to the vaccum podes on the controler behind your dash, these open and close the door to direct the air flow
More than likely the vacuum feed line from the engine has been disconnected or cracked. When there is no vacuum supplied to the HVAC it reverts back to the defualt position, the defroster.
Remove you glove box. It should be behind it.
behind the ashtray is a vacuam control soliend it has 5 coils that control the vacuam doors
The ECM and TCCM are located behind the lower dashboard on the passenger side.
Usually found behind/under the glove box or behind the kick panel.
what is recomended when replacing timing chain in 99 blazer with a 4.3 v6
Where is the control module located on the 1983 Chevy blazer
On a 1995 Chevrolet Blazer, you will find the central locking control unit behind the instrument panel on the lower left-hand side. The 1995 Blazer SUV with the 4.3 liter engine gets 17 miles per gallon in the city and 23 mpg on the highway.
i believe it is for the IAC valve. (idle air control) valve. cleaning with brake cleaner can solve the issue. or replacing completely.
The air door under the Blazers dash could be stuck, or there could be a vacuum leak on the engine. The door opens and closes from vacuum, directing the air from heat to defrost.