Heating problem Ford Taurus

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hi I just got a 1989 Ford Taurus and it has a heating problem. It won't heat the inside unless you are parked for awhile, as soon as you start to drive, it cools off. We thought the heater core was plugged but we took off the hoses and blew air both ways and there doesn/t seem to be any obstruction. Is there some problem with the doors in the heater box? I am beginning to think that the vent door is not closing and letting the hot air in. Can anybody help