04 buick rainier ac stays on defrost after battery replaced.

Are you saying the A/C turns on when you change the mode to windshield defrost?

No when I turn it on it doesn't switch to front vents.

May need to be initialized. Don't know for sure. I know Toyota'* a lot of stuff need to be after battery being replaced. The computers lose their memory and gotta relean.

Who changed the battery? As with most of the OBD2 cars a battery minder really should be used to keep all the old settings including radio & BCM & TCM.
Edward, Does the Rainier have an off button as selection with defrost, if so try tuning it off, disconnect the battery ground for a few minutes and then reconnect the ground.
If that does not work you will need to get the vehicle'* codes scanned for the BCM & if there are any air conditioning codes.