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Your association with Germany is correct. Not only is "D" the country code for Germany, but also the car itself is of German origin. It is a "Trabant" (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 as well). After Germany's reunion in 1990 the owner replaced the East-German country code "DDR" with the West-German "D". 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. 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.