Dont forget to clean the cabin filter also. Could be a mouse nest in the duck work. Good guess the heater core could be clogged. Start the car and see if the hose going to the heater core is getting hot. 1 should get hot. If not you may have a valve thats not opening. Try moving the controls inside and at. The same time someone try moving the control valve. Where is it you may be asking. Follow the heater hoses. If it has a valve on the hose and the hose gets hot on one side good guess its the valve. The valve stick or break so trouble shooting till you find the problem.

If your coolant isn't low, it could be a stuck thermostat or a plugged heater core