in the W124 body - the cabin air intake is in the wiper drain cowling, its the two grilles to the left and right of the wiper base. This area is positively closed off from the engine bay. There is a rubber weatherstrip for this job on the edge of the false bulkhead.

Are you mentioning the front piece that clips under the hood?

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)