While there are several ways to approach this task, here’s how I proceeded. If you wish to duplicate the procedure, be sure to follow it exactly.
Make a copy of the body and place it on Layer 0. Turn OFF the visibility of the “body 2” Layer. Go to Isometric SE view. Activate the Facet Editor tool and hover the cursor over the left side of the body. Click the left mouse button when that face of the body becomes outlined in green. Go to Modify>Modify 3D Objects>Create Surface>From Face(s). The rest of the body will disappear, leaving only this one face.
Click the Selector 3D Properties icon on the Inspector Bar and set both Extents CS and Inspector Bar CS to “User CS.”
Switch to Front view and Workplane by View. Draw a 3D Box, 36mm in the X direction, 13mm in the Y direction, and about 10mm in the Z direction. Still working in the Front view, place the Box where you want the sign to be on the body. Go to Left Side view and move the Box in the Z direction so that it intersects the body.
Create a new material, using “Wrapped image” or “Wrapped filtered image” Pattern, choosing “XXXX Sign3.jpg” as the image. Use “Arbitrary Plane” Wrapping, with Scale = 36; Aspect Ratio = 0.36; Origin = -18,-6.5,0; Normal vector = 0,0,1; Up = 0,1,0. Leave Transparency and Texture at “None” and use whatever Reflectance you like. (I used “Plastic.”)
Open the Materials palette and assign this new material to the 3D Box.
Select the face you made from the car body, right-click in the Materials palette, and select “Set ‘None’.” 3D-Intersect the Box with the car body face, being sure to click on the Box first. The result will be a rectangular “sign” that conforms exactly to the shape of the car body.
Move this new object 0.1mm in the Z direction of the current Workplane so it won’t be buried in the car body. Restore the visibility of the “body 2” Layer and place the new object on this Layer. You can either Group the sign with the body or 3D-Add the sign to the body. If you use 3D Add, be sure to click on the sign first.