Let me start by saying that I don't know the answer to this question, I (thankfully) have never had to touch that are of my X5 - but is it possible that something has "invaded" the fresh air intake and has the flap obstructed, or is the fan in the way? Any chance of getting a usb snake light in there and taking a look? Cabin air filter would have to come out minimum, and maybe the bulk-head if you really wanted to get in at it easily - if it's possible?