I cant speak for the OP but I am understanding his question as the fin that goes from the cockpit to the rear wing on GT1/GR1 cars. I have also tried this on the TS030 and I haven't been able to figure it out either
Use the same method. I just gave it a go on a TS050 and can confirm it works. Actually I'm really impressed with the mapping of decals on these cars, it's amazing being able to use this option.
So the process again to put your decal on the surface you want:
>Select "body"
>Add layer
>choose a camera, it actually doesn't matter which one,
>Projection method > Align with camera
>Rotate your camera to point squarely at the surface you want, like the fin or whatever. Use R1 + thumbsticks to rotate and pan, as normal. Use L1 + left thumbstick to target where you want to fire the decal. You dont need to click anything when you're done, just leave the camera in a nice place and then go to select your decal.
>Select decal and it'll go right where you targeted.
>Make sure to check the back side from where you placed it in case it "bleeds" through, and go "set projection area" > "Angle Limit" and turn that down to back that bleed off.
Same deal with pasting decals on bumper corners. The editor is really well featured, we just need to learn how to use the tools to make cool paints.