Google sketchup, interesting free 3D modeling software, not the most powerfull, but Youtube has some good tutorials.
Google SketchUp
Car blueprints, well sort of, you might have to take a measurement or two to get the scale close. You can also get trucks, trains, plains, X-Wing fighter...
Oops that last one was my inner Geek coming out.
Blueprints - Cars
One of the tutorials, it isn't going to give you a perfect model, but it will get close.
It should be able to get you a close model of your car so that you can then model alterations, and its all free.