2004 Tahoe - rear hvac system only blows cold air (diagnosis and temporary fix)

This Tahoe has the auto front and rear heating and air conditioning (hvac) systems. In this case, the rear system fan works, and you can change airflow from floor to ceiling. The rear only blows cold air.
This video covers the temperature components of the rear unit. I test the coolant temperature at the rear heater core, the temperature door, the temperature door actuator, and the rear aux hvac controller motor control circuit. All of these tests can be done without a scan tool.