It may be where the cabin vent is but I think it is shaped with the louver to help with aero. Let out the air in a lower pressure zone. My car also has the wrap around areas on the windshield. My guess is that they were improving the aerodynamics of the car as they made them.

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.