2006 Volvo XC90 seem to require rear-AC on to have air come out the vents for 3rd-row seats - defect or design flaw?

Answer 1

Unfortunately, that's the way it works in my 2006, too. Use the fan control that is at thigh-level to the left of the third-row seat, driver's side, to adjust the flow rate through the rear vents. This control only works if the both the "AC" and "Rear AC" switches are activated. It would have been nice if the rear blower could contol fresh air flow also. Not really a "flaw", per se, just short-sighted design.