The pathway which the air is directed is controlled by small vacuum lines. There is a main vacuum " Feed" port on the intake plenum; it is usually smaller and of a harder plastic-type material than the normal rubber vacuum lines which are used for engine emissions.
Follow this line from its start point to the passenger side; look for any broken or kinked areas.
Also, while the following requires some disassembly of the dash, there are vacuum lines which run to the back of the selector switches. Make sure the vacuum lines are attached. And play around with them: Detach them, plug them with little rubber grommets or whatever you have laying around. See if it makes a difference to where the air is directed (which vents).
There is also a damper control on the airbox (under the dash, passenger side). See if it is moving (the lever) depending on selector switch position.
All tests need to be made with engine running. Otherwise, there wiill be no vacuum.