A picture would be good. I have trouble imagining that GM would put a heat stove and hose on the air cleaner...and NOT install a flapper.
But unlike the older ones, the GMT400/TBI flappers are not vacuum-operated. They've got a wax lump inside a mechanism, similar to the coolant thermostat. When the wax is cold, the flapper allows hot air into the snorkel. When it gets hot and expands, it shuts off the hot air. In-between, it's regulating the air temp inside the air cleaner.
And the whole thing is hidden, internal to the air cleaner.