Just a few thoughts.

Have you checked Defrost Mode at the front windshield? If you are getting heat in and out of the heater core then you should have some heat, Defrost would be a good check.

Also Pod#40 has a physical rod connection between it and the center vents, the pod may be bad, or the rod was not connected. The thumb wheel on the center vent controls the upper flap, and the underside of that has a metal rod attached to the lower flap on the heater core to divert air to the center vents or the defroster. If this has been worked on in the past, something may have been left disconnected.

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)