no rear heat on 2003 expy

on my 2003 expy, the rear will only blow out cold air. Any thoughts on what might cause this?

could be a faulty temp blend door actuator on the rear hvac unit. do you hear the doors move when the temp is changed from cold to hot?

could be blocked rear heater core or low coolant to the rear heater core. grab the hoses under the vehicle for the heater core. Are they hot when the engine is fully warm? Working properly, with fan on high, there should be at least 10 degree temp drop from the inlet hose to the outlet hose.