This group is about the ducting and the outlets rather than the blower motor or the heater core, which are listed elsewhere.
The common failures are a broken louvre, a vent that will not stay in position because the friction tab has worn, and a duct that has come apart at a joint and now blows air into the wrong place. On older vehicles the foam seals around the duct joints crumble, which produces a rattle and a loss of airflow at the same time.
Order by position: which outlet, which side, and whether it is the upper or lower duct. A centre vent housing differs from a side vent housing even where the louvres look the same. Where the vent includes an airbag or a sensor, that configuration is part of the part number. Ducts are bulky but light, and they ship as parcels.
