1997 Buick LeSabre - Unbalanced heat

Left side of all a/c / heater vents is cooler than right side whether on heat or vent.( I think outside air is leaking in to left side?

Sounds like a blend door problem . Sometimes you can do a reset procedure . I would do a search on youtube on how to do this , If you have to replace the blend doors this can run into a lot of money because a lot of times you have to take apart part of the dash to replace these .