It's my opinion that the amount of air coming out of the rear passenger air vents leaves much to be desired. The fan speed has to be turned on quit high to get anything coming out worthwhile. Anybody have this issue?
Yes, my kids in the back seat complain about this. In addition to setting the fan to the highest setting, it appears that the vent controls have to be set to blow horizontally. Blowing either down or horizontally/down don't produce much air flow to the back vents.
Make sure the main HVAC setting is on the center vents if you want those rear seat upper vents to work. In my car, the HVAC auto setting sends all the air to the lower (footwell) vents and/or the defrost vents most of the heating season.