I used to have this problem as well. I found the answer, for mine anyway...
Take off your glove box and pretend you are going to change the cabin airfilter. There is a narrow access door that comes off allowing you to change the filter. Above where you open the door to access the filter, you will see three holes with rubber flappy things covering them, the flappy things are on the inside of the air tract. When you press air recirc, a valve blocks the incoming air tract and now the air is sucked from inside the cabin, and the rubber flappies move inward as the result. Make sure all the flappy things are covering the holes, and that none are trapped in the holes. Next go up a little further and you will see an accordian type bend in the ducting that angles towards the firewall. That entire angled accordian bend can move upwards and expose the air tract from above, and if its not pushed flush with the solid piece below it can leak air like crazy. Sometimes items from the glovebox bump this angled accordian piece and dislodge it from making a tight fit into the solid piece below, and there is some wiring that can get trapped in between preventing it from making a tight fit as well. I suppose if it kept coming loose one could duct tape it so it couldn't become dislodged as well. Once you push down on the accordian piece so that it makes a tight fit the air leak problem should be rectified.
'09 Corolla CE Enhanced Auto
TRD Springs/Sway/Xrs FSB by Yamaha
Lifetime : 6.111L/100km(38.761mpgUS)
(manually calculated) (original unflashed factory ECM code)
Last edited by LeanBurn; 11-16-2009 at 07:37 PM.