Sounds like a stuck mode door on the right side. Look up an HVAC diagram for your make/model and see which mode doors control which vents and isolate the one that affects your right side. These 'doors' are just plastic flaps, but the actuators that move them are little plastic geared motors that break down. If that's the case, you'll have to get to it inside the dash. Some cars that's easy, some it's a major PITA. Of course before you rip a dash apart to replace an actuator, just run a simple voltage test at the control to make sure it's not a bad switch.
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