i have a problem with a ford escort n reg. the heater is blowing cold all the time, i have replaced the thermostat. and still cold air coming from the heaters in the car. there is hot water going to the matrix and returning to the engine, my question is what could it be the is a thermostat senor switch connected to the thermostat housing could it be that or something else, engine keeps good temperature when running. no overheating problems.