Check the airflow thru your heater core, and make sure the doors are closing in the airbox.
Most American cars have full-time hot water flow thru the heater core, and just divert the hot air to outside when not needed.
Usually two doors, one for floor/defrost, and other for outside/recycle air need to be checked as well.
Its real easy for a door to not close fully, and also for the heater core fins to get plugged with dirt, dust, etc.
Finally, it may have a cabin air filter that might be plugged up.
And when its real cold, covering the radiator 75% sometimes makes a difference. Just watch your temp gauge, and listen for the fan to come on. Ideally, you want to be off the thermostat, as in full open, and before the fan comes on.............