The thermostat would be the place to start if you are sure its not the heater core. Heater core would have tell tale signs like antifreeze mist blowing in the vents or wet carpet soaked with anti freeze on the car floor, usually tword the passenger side. If its not the core, or the thermostat when you change it, then its probably in the hvac controls, but the core and the thermostat should be checked first. Also let the car warm up and feel both of the small hoses running into the firewall to the heater to see if they are hot. If not, one might be plugged. Hope this helps, Clay