[jdurant]

I just dealt with that problem. Mine was the gasket that goes around the flapper door that closes for max and opens for normal a/c. Thats only on the passenger side. It's part of the kick panel trim piece.
But since yours is on both sides, I would think windshield gasket, maybe. If not the windshield gasket, the first thing I would do is take off the cowl panel and take a look in there too.