There are two parts to the rearlights there is the actual lens (glass) and then there is the plastic cover that holds the bulbs, complete with the wiring, against the lens. The euro and US lenses are the same but the wiring is slightly different due to the side marker lights.
So, because the wiring and the back cover is seperate from the lens you can change the lens as you see fit with anything from europe and vice-versa.

88 230E (W124)