The air should come most strongly out of the center ducts in the A/C position.

You probably have a bad vacuum servo to the center duct or defrost duct. They are on the same leg; one opens and the other closes on the same signal. For safety sake the system defaults to defrost when the vacuum system is failing. Probbably not a DIY job.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician