Weird A/C Issue
 

The air conditioning in the wife's '05 blows ice cold in 3 out of the 4 vents. The inboard passenger side air vent blows the same cold air as the others some of the time but usually it just feels like outside air coming in. It happens at idle and on the road. I haven't messed with A/C very much, where do I start and what should I be looking for? Thanks in advance!