If you don't need the cake too soon, you can order Mike McCarey's DVD, "Cakenology". It's an awesome DVD that helps you with realistic car cakes.
If you need instructions sooner, talk to Bellatheball or go to her page on Flickr; she gives instructions (they are hilarious and awesome) on doing a car cake via Mike McCarey. http:[email protected]/3587804466/
Not sure if you can make a carved car cake that will feed 10-15 people, i'm thinking it may need to be bigger.
I've had a look at the website and it's helped a lot.