I think you need to replace Mode 1 and Mode 2 doors if you're not getting airflow from other vents (defroster, floor). Those are a ***** to replace; they're behind the center stack and necessitates pulling the dash. It's a costly repair, so I would recommend doing it once and replacing all the doors. You can additionally test the actuators, but they seldom fail. To test them, get a 9V connector and battery. Touch a lead to each pin on the disconnected actuator. 9V is sufficient to make them move and they're not polarity-sensitive.
There is the HeaterTreater repair method (search for it here or in a browser). I would not advise hacking the plenum and patching with tape; even so, there is a bracket behind the center stack that would have to be cut and patched. However, I would recommend HeaterTreater doors. They are costly, but the last ones you'll buy.
Regardless if you pay to have it done or DIY, do consider changing out the heater and evaporator cores while everything is open. You'll have to disconnect A/C and heater lines to remove the plenum, anyway. There are plenty of tutorials here and on YT if you want to try.
