This sounds really weird but with the AC on and rear vents on, the rear passenger vent frequently blows out warm air. It doesn't happen all the time, but more often than not. Meanwhile you can clearly feel the difference with your hand to the driver side which is actually cool. Maybe this is just some weird effect of the positioning of the front vents, but issue still remains that it is definitely blowing out warm air in the rear. My daughter gets hot back there. It ends up being better to just turn off the rear vent.