The a/c in my '03 mercury moutaineer blows warm air on the drivers side vents, but cold air on passenger side. The dual center vents are half warm and half cold. Is a duct or vent stuck?
Quite likely.
Maybe a disconnected or stuck blend duct door.
Are your heating/a/c controls vacuum operated or linkage?
If vacuum, make sure there are no damaged or disconnected hoses.