16Y.O. Ford?? hmmmm..... is the hot water in/out hoses hooked up to the heater core? If the core was leaky , someone may have disconnected it. It could also be a vaccuum leak , causing the vent ducting not to change from cold air to hot air.

 

some of the cores in the heater core could be clogged.

 

has the heater core been bypassed ? have you checked to make sure the heat blend door vacuum check valve is operating properly ?

 

has the heater core been bypassed ? have you checked to make sure the heat blend door vacuum check valve is operating properly ?

were is the heat blend door vacuum check valve

 

Did you put the thermostat in backwards? Did you even check it in boiling water to see if it opens before you installed it? Is the thermostat opening when the vehicle warms up (coolant flowing through the radiator)? Are the hoses hooked up to the heater on the firewall?

If the core was leaking..someone may have bypassed it.