Are you sure it's ALDA? Might it be the transmission's VCV. It draws air as needed from behind the dash because (in theory) it's cleaner than air under the hood.

Could it be the line from the shut-off valve on the IP to the ignition switch?

There are also one or two vacuum vent lines from the EGR solenoids. They also draw clean air from behind the dash.

With this type of question the OP is advised to include a photograph, for example, this one showing the VCV:

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
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"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970