If you want to do decals on the underside of the wing (as viewed from the rear) there are 2 methods.
Method 1) using the place decal with camera method, spin the car around with the camera to the rear and place your decal. Adjust using depth and angle. Its downside is it's more difficult to get the decal straight compared with method 2.
Method 2) place your decal on top of the wing, eg BMW M6. Then use the reverse decal function. It will now be reading backwards. Use angle +10. It has now gone through to the rear and will read the correct way. Adjust accordingly. Now place a rectangular decal over the other decal in the original colour of the wing. You can then place any other decal over the top. Dont use angle or depth on that one. This method doesn't work for special colours, in which case use method 1.