Many thanks in advance

 

There's just not a lot of air going back there.

 

So what is happening in the back? Does it blow air, but it's hot outside air that has not been cooled? It doesn't blow air in the back at all (hot or cool)? Or you can feel cool air, but it just barely makes it to the outlet vents, no air flow? You could have an air management problem if you get nothing out of the back vents. There is a valve system that opens and closes flaps that control whether the air is hot, cool, or neither. There are valves to open/close air flow from the heater core, and from the A/C cooling coil. If the rear seat vents have their own valves, it's possible you have a failure of one or more diverter valves. But I have never seen a schematic of the air flow system so I can't tell you how this system works on Cayennes. But perhaps someone else who has worked on diverter valves can fill in some blanks.

 

There are only a couple of small vents into the back of the truck, but there's a huge air mass back there, and a lot of glass. I just don't think it can keep up.