Okay so every vehicle has 2 files that make it a vehicle:
The wft is a 3D model (gives the object its shape, proportions, etc.)
The wtd is a file that contains all of the textures that go on the model to give it colour and essentially something to look at.
The wtd file is basically a number of images that are mapped onto the 3d model and each part of the 3d model has a 'layer' in the wtd file for its texture.
Depending on what car you are editing, the image layer that is used for the main body texture of police vehicles is normally called police_sign or police_sign_1/2/3/4 if there are variations.
When people make custom skins for cars they normally just edit the police_sign texture so the car has a different paintjob and decals. You can see them in the gallery section of this website fairly frequently.
Other times, people who make custom textures will release the entire .wtd file which is much easier to replace.
So if you want to make your own textures for a vehicle, you need to use sparkIV to open the texture file, then find the image layer that you want to edit, then export it and save it onto your computer, edit it and then save it as a .png then import it back with sparkiv to replace the original.