rears are half texture and half skin. front is texture and i think hardcoded. this is why theres such a big difference between front and rear. paint the square black. itll darken it more and looks alot better with black skins on. the square is part of newer files and was not always on the texture files. today i think all cars have it. i recommend going black square on FXO too as that makes it look like a car with nice lines matching the rest too. fxo is very visible when changing the square.
just inside the square theres a thin ring along the edge which covers the numberplate edges.
i used screaming yellow paint to figure this out and made my numberplate edges on LX bright yellow, but not where text is. only back and edges. so even the square covers multiple areas inside the square alone.
attached 3 versions , one with yellow. one black. and one fully black. also original if you want to change back to original.