Might be a blend door problem (this assumes the blower motor is working). Get the engine up to temp and feel the heater core hoses to see if they are warm/hot. If both are warm/hot then the heater core is good. Move the temp selector from hot to cold and back, can you hear the blend door move? Feel/hear a difference in the air moving through the AC plenum box? If no I'd troubleshoot the blend door starting with the selector switch, making sure I had voltage on both sides of switch. When you're confident that is the problem Google a how-to guide on changing the blend door.
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