Auxillary heater not working on 1995 Ford E-150

When I turn on heat in front and rear I don't get any heat from the rear vents only cold air? I have already replaced the heater core at the firewall due to leakage issues so that is fine. What is the next step in diagnoses?

1 answer
There is a blend door motor back there to turn from cold to hot. This motor is likely not working.

Naturally verify there is no air pocket in the system either -- the auxillary systems are hard to purge indeed.