where is the hvac module located ,and what is it's function

Customer Concern: No heat. Tests/Procedures: 1. Verify engine is at operating temperature. 2. Check operation of hot/cold blend door. If in question disconnect the linkage and manually move the door to feel it seat in full heat position. 3. If the door is going to full heat, verify good coolant flow to core, if OK replace plugged heater core. Don't understand what you call "hvac module". You do the test above and you'll find you have a plugged up heater core