Heater air flow restriction
 

Wierd thing in our '92 HVAC system. When we turn the fan control to even highest setting, the air flow we get is a 3 or 4 at best. I suspect a damper or air control door of some sort, likely vac actuated. Ducts are clear. Haynes' or Chilton manuels do not even address vac issues as related to air flow control. Ideas?