heater only blows cold air, all the flaps seem to be working.
heater problems on 2003 Toyota Matrix
I am not sure but there should be a heater control valve usually in the engine compartment at the rear on the firewall with two hoses attached to it and a mechanical cable that attaches to the heater control knob or lever inside the car. Get a friend to operate the lever inside the car and see if the valve fully open out side in the engine compartment. If its low heater output perhaps the cooling system thermostat has failed.