air is cool and flows out of defrost and floor vents but does not flow out of the
main dashboard vents.
Do you have it on defrost rether than AC? Should it flow out the main dashboard vents?
Seriously, in some systems when you select some option such as defrost the system chooses the vents for you. Mine does.
Perhaps the owners’ manual has a description of the system’s operation.
Also, Ford uses vacuum from the engine to direct the air flow doors under the dash. If a vacuum line is broken or cracked, or a vacuum motor leaks, it could prevent certain doors from moving and re-directing the air flow where you desire it to go.
What year GM? Seriously, they have been making them for over 20 years! Does this have EATC (fancy-pants automatic/electronic controls) or manual (knobs) controls? Busted is right, this is vacuum-controlled in late models, at least. One door diverts to defrost and floor in one position and panel vents in the other. A second vacuum motor and door selects between the defrost and floor. It sounds like you have a problem in the first one.
You may find text and diagrams here to be useful: