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It sounds like you have a bad dual heater valve or a problem with the electrical circuit. What you can do is pinch off the hoses going to the heater core. If the air is cool on both sides then you know it's either the valve or the circuit. If you do not have much mechanical experience the valve is not an easy job.
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ASE Certified Technician
12 years at a Ford Lincoln/Mercury and Jaguar dealer as a technician and shop foreman.