The damper door seals have foam on them to ensure good airflow through the selected vent tubes, but after 21 years the foam breaks down and doesn’t seal tightly anymore. So not really much you can do short of tearing down your entire ventilation system to reseal. This is assuming all doors are opening and closing correctly.

 

Blend door malfunction or even mouse nest blocking air flow. You can remove the fan motor shroud to inspect.

 

On my 1999 Camry my blend door motor and mechanism got jammed due to wearing out I believe. So it was not diverting the air to proper places. If I remember it was stuck on defrost only. I had to purchase the part thru the dealer and replace it. Maybe cost under $200 I think. I suppose you could go to a junk yard and harvest this part, up to you. Sometimes you learn more at those junk cars on how it is installed. But the life of a junk part is sometimes unknown. Somewhat difficult a job to get it correct when reassembling the motor and blend door guide. Take note of all the things you do so you can reassembly it to work properly. The blend door motor is behind the center covers so you need to take some stuff apart and its up a ways on the drivers side right under there so get ready to stretch your body. Best of luck. Let us know what you find.