2002 century airdoor problem?

Howdy. 2002 Buick Century with manual, dual-zone HVAC, 128k miles. The passenger side works fine, but the output from the drivers side vents does not get as cold as the passenger’s when cooling. The slide control for the drivers side does seem to work to make the output warmer, but it never gets as cold as the passenger side even at maximum cool setting. The system also seems to be directing too much cold air to the defroster; I sometimes must run the wipers to clear the condensation off the windshield at low speeds. Any advice appreciated-I don’t want to tear into the wrong part of the dashboard and am unsure where the offending door is located, if that is indeed the problem. Thank you.