The Auto climate control has a mid setting & that is what gets it heading though the vents.
The top right button for defrost is what gets it heading to windshield.
There are Total of 4 actuators, pass & drivers side, (mix door actuators) air inlet actuator and the mode door actuator as you mentioned.
Greater info can be obtained by connecting a Tech2 scan tool for codes for movement faults/diagnostics.
I may have to spend some time in the FSM to identify where exactly the mode door one is located.
However, Possibly one of our members who is more familiar with the HVAC system/auto climate, may be able to help you further. Hope this help for starters..
1997 Buick Pk Ave (Soft Ride) Suspension!