Does the water temp gauge on the dash get up to indicate the car gets up to temp? If not, it may be the thermostat.
If the gauge goes up to normal (about 1/2 way - 90C) then its most likely the electronics in the dash, or more so the motor that guides hot / cold are around the heater matrix. With the ignition on, can you hear motors working behind the dash when you move the temp control from hot to cold?
What If There Were No Hypothetical Questions?