I experienced an issue with my mini-truck where some fixtures like exhausts and some lights showed some weird behavior and I think I found the problem.
It seems like the game has issues finding which side of a surface is outside of the car when the surface is perfectly perpendicular to the Z-axis, and tends to stamp some fixtures inside out.
Fixtures appear like this if placed directly on those surfaces. When placed on curved surfaces or on edges, they appear fine.
If you place the fixture on a curved surface and then drag it on the flat surface, this problem does not appear.
Also, in the case of the pre-wedge supercar, it only seems to have that issue with exhausts, but not with lights. For the minitruck it happens with some lights.