Your association with Germany is correct.

Not only is "D" the [country code][1] for Germany, but also the car itself is of German origin.
It is a "[Trabant][2]" (aka "Trabi"), which was produced in East-Germany and the most common car there.
It became a symbol for the fall of the inner-German wall, since many East Germans explored western Europe with the Trabant (there is a [movie about that][3] as well).

After Germany's reunion in 1990 the owner replaced the East-German country code "DDR" with the West-German "D" (or just put the new sticker on top, as it seems to be the case here). 
Although many people sold their Trabi as soon as possible and bought a western car, the Trabant was quite popular for others and [spread to the western world, including the US][4]. That explains its way to South Carolina.

Why this car was in that scene, I can not tell. 
[ An easy explanation would be, it simply looked exotic to the producers. 
I could also speculate about deeper meanings and an analogy regarding the East bloc equals hell/purgatory and the western world being the land of the living, but that would go too far.]

**To sum it up:** I think, there is neither a deeper meaning in the sticker itself, nor a continuity error here.

  [1]: https://en.wikipedia.org/wiki/List_of_international_vehicle_registration_codes
  [2]: https://en.wikipedia.org/wiki/Trabant
  [3]: http://www.imdb.com/title/tt0101960/?ref_=fn_al_tt_1
  [4]: https://en.wikipedia.org/wiki/Trabant#1990s_and_beyond