It sounds like your problem may be in the heater control valve which is located under the hood in the engine compartment.
This vacuum operated valve stops the flow of coolant thru the heater coil when it is closed.
It is located on the left side of the engine under the intake manifold toward the rear of the engine.
You will see the heater hoses entering and leaving the valve.

This valve is operated by a vacuum signal that comes from the heater control inside the car.
If it is closing properly, it won't allow any coolant to flow to the heater.


hope this helps
steve