Check under the hood near pass. side fender and along fire wall for a broken vacuum line ... may be easier to HEAR a vacuum leak than to see it.(hard pre-molded plastic line --not rubber hose-- very brittle with age& heat) may be taped/wrapped with other lines and covered by corrugated tubing. that's why the listening part is suggested.
First you need to find out if your heater control, is electronic, cable or vacuum controlled? You can remove the control from the dash and look for cables or wires. Next be sure cables, wires or vacuum lines are all attached to the control. Next check to see if the cables, wires etc. are attached to the damper door arms on the heater box under the dash. Move the selector to the differant positions and watch for movement of the arm, check for voltage to electric accuator motors. No movement or voltage repair selector switch or cable, movement be sure motor is running if not replace. Ifall this checks out look for broken damper doors. or leak in vaccum lines.