Why do my A/C & heater only blow from the defroster’s vents?

My A/C and heat both work, but the cold and hot only come out the defroster vents on top of the dash.
Moving the switch makes no difference.
Is this switch electric or vacuum? Could a vacuum hose be off someplace?
Where’s a good place to start looking?

The vent doors, usually found inside the dash area, that serve as valves to control the cold and hot air flow are not being activated. They are stuck in the defrost-mode position.
Therefore they do not move to close and open the lower doors leading to the regular A/C and heater ducts.
Identify whether your vehicle has vacuum-activated doors or electrically-activated doors by looking at the control head.
If electric, check fuses and for loose electrical sockets and wires.
If vacuum, check for one or more leaking or disconnected vacuum lines. These sometimes come loose from the control head.
Start checking for these conditions at the control head.

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