I will explain it here as best I can. but it's not a simple process with simple tools because of the way the game is set up now. Used to be you could use ninja ripper to your hearts content by simply getting the vehicle on screen and hitting the rip button but it's strange now how the game renders Geometry so you have to do it just right or it won't work. I am using the steam version myself. I don't know if an other versions on pc work differently but this is my tutorial for the steam version as it is right now.
So you can still use Ninja Ripper with direct x 11 injector (nothing else will work with the game) but you have to hit it just right to make it work otherwise you only end up ever ripping the garage Geometry.
Basically while in the garage vehicle viewing area. You scroll over to the car you want, Let it load in and then start to select a different car, while the cars are switching that's when you have to hit the rip button. because it temporarily renders all the geometry including the vehicle in a way that Ninja ripper can fully rip. Do it any other time, including in a race or derby and it won't work. I hope this is clear enough. basically with the right injector you have to be in your garage, select the car you want first and then select a different car and while the new car is loading in press the rip command keys in ninja ripper.
I'll try to clear things up more if people still don't understand I am sorry if this doesn't entirely make sense but I have gotten full Geometry from any car this was on the steam version of the game it's very tricky but if you do it right then it should be easy to rip ANY vehicle from the game with all it's parts.
I went back over my logs for my successful rips and here is the version and injector I used
Ninja Ripper 1.7.1 x64
Inject mode: "d3d11.dll wrapper"
Using these settings on that version I was able to rip Geometry as written above.