heat! on 1993 Nissan Altima

I have no heat! replaced resistor, all fan settings work, a/c works. Coolant is at proper level. does this just leave the heater core or is there something else i can check before that huge job and expense????

Asked by for the 1993 Nissan Altima
Make sure your heater control valve is actually opening.
where is the heater contriol valve located