Is the heater control valve closing? Pinch off the heater hose the the valve and see if you get cold air from the sides. If so, check for either a vacum leak or a busted diaphram in the valve itself.
The valve is under the cowl cover on the r/h side, sometimes the vacum hose is routed under the battery tray and will be damaged by battery acid.
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.