Okay I faced same issue in my brand new fortuner 21 sigma FL , fortunately diagnosed it while taking delivery at Toyota Rawal
The issue was that the, 3rd row right seat which folds up has a vent behind it which sucks in air , the plastic cover(shoper) of the s3rd row seat was sucked to the went and was not letting air flow inwards , as soon as we pulled the seat plastic away air came from 2nd row seats, so try that and let us know.
The vent is here
