heater doesn't always work on 2001 Toyota Celica

The heater doesn't always work. Sometimes it will come on after driving for a while. I have checked the water level and the leaving water line out of the heater is cold. What could it be?

1 answer
Does the engine warm up completely? I'm wondering about the coolant thermostat being defective and allowing the engine to run too cold. Another thing is that there may be a problem in the heater control module in the dash or that the blend air door actuator is not working. It's also possible that the heater core is plugged or there's a blockage in the lines to or from the heater.
You may need a Toyota specialist for some help on this.
Here's a directory link for you:
http://repairpal.com/directory?address=77845&car_brand_names=Toyota