I don’t know which vehicle outline library you are using, so this is for Impact, although others are likely to be similar.
Apart from the body lines, the drawings have a line going around the complete perimeter of the vehicle. That’s there so that we can change the body colour of the van quickly. What you need to do is to select that line and go to the colour palette in Illustrator. You will find that the line is given a stroke already and all you have to do is make sure that it has no fill.
Copy and paste work in exactly the same way as they do in Photoshop. I’m a Mac man so I don’t know the PC shortcut, but with that perimeter line selected go to the edit menu and choose "copy".
Then select the layer with your image, edit menu again, and select "paste".
That will give you a copy of the vehicle perimeter on top of your image. Select both the perimeter line and the image, edit menu again and select "crop". Then only the part of the image that falls within the vehicle outline is visible.
Keep both items selected and move them over so that they line up directly over the top of the original vehicle outline, and job done.
OK?