That's easy. The damn defroster. I live in a hot humid climate. I do not want more hot air for defogging on humid foggy mornings or nights. I want cool dry air blown on the windshield.

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Remember, Safety Third!
'99 E300 Turbodiesel, '82 300TD, 1996 12V Cummins Turbo, '94 Neoplan - Detroit 6V92TA