In order to model accurately I need reference images illustrating the shape of my vehicle from the cardinal directions: front, back, side, top, bottom. Blender allows me to put these reference images into the background of the editing views, helping me precisely shape the model.
The best format for reference images, if you can find it, are blueprints. In theory, you can also use well-framed photographs, but in my (limited) experience blueprints are easier, as they don’t suffer from perspective (shapes getting smaller with distance from the camera). Here is my current collection of blueprint hosting sites. If I’ve missed any please leave a mention in the comments; and if any have stopped working also let me know so I can remove them.
This article on BlenderNation lists additional reference image sources for everything from spaceships to cartoon characters. You can also try using Google to search the listings of the US Patent Office. And if you have any good tips for taking photographs that can be used for modeling, please pass them along in the comments and I will include them in the article.
You will also need some specifications for the car. Width, length, and height and a minimum for modeling. If you plan to import your model into a simulation and actually drive it, the more information you can find the better. Here is a specification sheet I found for my car.