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The first thing that comes to mind is what is called the "Thermostat" it manages engine temp to allow the engine to get up to the proper temp for the coolant to go through the heater core, thus allowing the heater to operate. First suggestion is to have the t-stat replaced (regular maintence item anyway) than if still no heat, check the heater hoses(2) going into and out of the heater core and see if both are HOT to the touch,,if so the problem lies within the"blend" door in the heater box if one hose is hotter than the other that would indicate a restriction in the heater core. If you need instructions on replacing the t-stat just let me know,,,it should be done for maint reasons anyway. If not, It's an inexpensive replacement at any repair shop.
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TIP: Fill the tube with antifreeze before you install the t-stat.