I have a 1997 F150. Heater was blowing cold air. Temperature gauge was staying just below normal. Replaced thermostat. Truck is still running cooler than normal and heater blowing cold air.

 

Are your heater hoses hot in the engine compartment? If so, then you probably have a blend door messing up. Very common problem, if that is what it is. Youtube has a number of videos on how to do this without removing the dash assembly. You go in thru the glove box area. Check it out.

 

I will check. Thank you very much!