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Ok first, you need to make sure that coolant is actually circulating through through the heater core. Just feel the hoses.. They should both be warm, or hot to the touch. Also, the obvious.. is the engine warmng up? Most likely....?? If not, then put a thermostat in it.
Anyways, back to the heater hoses...
It they don't get hot, then look for a shutoff valve on one of the hoses, or a low coolant level. If all of that is good, then the heater coer is clogged.
Here is a picture of where they are under the hood:
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