There could be a mouse or squirrel nest inside the plenum.
As you thought, there are no air filters. However, there is a design element which could lead to a restricted airflow; the heater core and the air conditioner core are sandwiched back to back. If anything like for instance leaves, lint, or dog or cat hair got sucked up into the air intake, having the two sets of heat exchanging fins in line assures a 100% capture and partial obstruction.
You might also want to check to see if the actuator for defrost mode is sticking... That would divert a large proportion of the air up at the window. It's located on top of the air plenum behind the radio.
Are you aware of the push/pull function of the push button climate controls? If you only push the buttons, you get essentially a reciculate mode, which doesn't have all that much air flow. If you push the button in, then pull it back out, that engages the fresh air mode, which can be like a hurricane by comparison.
-It's been such a
LONG TIME... BlueShift>> 1981 Dodge Ram B250 Custom Sportsman Maxi Van
It's what you learn after you know it all, that counts...
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Learning from my own mistakes is good, learning from yours would be much better!