In my opinion it rarely is the interior evaporator, its always seal rings and stuff like that, or the evaporator in the front because its exposed to road salt grime and rocks .. my dad has trucks that passed a million miles and we never changed any evaporators inside the cab.. problems are always outside where the components really take a beating.

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1987 190E 2.3 8 Valve
1993 VW Passat Turbo-D