My 93 LE's heater has become very weak. With the heater control on full heat and the fan on high only slightly warm air comes out. I checked the coolant level in the radiator and it's full. I'm guessing the prime suspect is a stuck open thermostat but the engine reaches normal operating temp just like it alsways has. If the thermostat was open I'd expect the engine temp gague to show it running cool. Advice appreciated.

With the engine warmed up and the heater on full, feel both hoses running to the heater core. If one is significantly hotter than the other the core may be clogged. Reverse flushing it is usually the cure.
Also check that the heater control valve which is located on the firewall just to the left (as you face it) of the brake booster is operating freely.