In my 1999 ml430 when I turn on heat it blows cold air what can I do on 1999 Mercedes-Benz ML430

I changed cabin filter still blows cold air what's next

Asked by for the 1999 Mercedes-Benz ML430
could be blend door stuck open
What is a blend door???
It's a door inside the HVAC housing that controls the airflow through the heater core and A/C evaporator , to regulate air temp. Cabin filter - filters air. Stuck closed thermostat would cause overheating. CHECK coolant level first , and make sure the engine is reaching proper operating temp.
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Replace the thermostat, it is probably stuck in the closed position.
Where is thermostat