If tthe thermostat is OK then have a feel of the two coolant pipes that pass into the car through the engine bay bulkhead, both should be warm with a warm engine. If they're not then its likely the heater matrix is blocked and you should do a reverse flush on the cooling system to try and clean it out. This is done by removing the top hosefrom the radiator and placing a garden hose into the engine water outlet, and then run the hose until you get clean water coming out of the radiator.
If this doesn't help this only leaves checking out the motor / cable that moves the air mixing flap that allows the hot air from the matrix to enter the car air system.
This flap is controlled by the temperature knob / dial etc and I think is controlled witha small motor for your car so the thingto do is first get access to the back of the control knob and measure the voltages is present there, then trace the wiring back into the dash from the control to locate the motor.
It’s worth first checking the fuses for the fan / control circuit
in that case have you tried calibrating the climatronic flaps?
this video shows how