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

Where is thermostat

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.