This article is from the VW Technical FAQ, by with numerous contributions by Jan Vandenbrande others.
A: That's a "Nebelschlussleuchte" (sp?), or rear-fog anti-crash light. Many
of the newer cars shipped to the US have these incorporated (but not
connected) into the rear lenses [Corrados, Passats, Audis]. Very effective
in fog, snow and heavy rain.
 
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