Manifold versus non-manifold meshes
Before going further, there's an important topic we need to touch on. Right now, we can see the back side of our body panels from certain angles. In other words, when you look into the wheel well, you can see the back side of the faces that make up the truck's tailgate.
This generally isn't a good thing, because it reveals the single-sided nature of the faces (and can sometimes have odd effects on your shading and materials). However, you can often get away with it in Blender. If you happened to see the back side of a body panel during an animation, it would be hidden in shadows and probably wouldn't look too bad. This is especially true with a low-poly model like this, where you're not expecting perfect realism.
The problem is that some game engines aren't as forgiving as Blender. To increase rendering efficiency and reduce CPU/GPU workload, they simply don't render the back side of polygons. Instead, it will appear that those faces are missing from the...
