Is your anti-freeze level full, thermostat working properly, bad radiator cap not letting the system pressurize? It is either your heater core or blend doors I would assume. Go to http://heatertreater.net
, they have videos that show you how to check the blend doors to see if there is a problem. If thats not it, you might have a plugged heater core. Try flushing it with a water hose and see if anything comes out of it. Hope that if it is a plugged heater core you are able to flush it, if not and you have to replace the heater core you have to pull the entire dash out to replace it. Be sure and bleed the system after you put everything back together to get the air out.