2006 Chevy Equinox doesn't have heat

Replaced the door that is in the dash and the thermostat. Still doesn’t have heat. Very frustrated, this car hauls my grandkids around and the money invested is over $2000. We don’t think the mechanic knows what the problem is and keeps trying things. Anyone have suggestion? My husband knows quite a bit about cars and it doesn’t appear to be the heater core. They also checked and added coolant without issues