David,

You wouldn't have to do a patent search at this point...generally speaking the whole car isn't patented, just various components of it and any unique features.

The car companies don't usually get involved with independents. They have their own concept car groups internally where they will develope ideas. If the car is going to be of limited production and they still feel interested enough they will contract with an outside company to build it. This is how the Ford GT came into being.

In the Detroit area there are a bunch of these companies, most of which I don't remember the names of but one I do remember was Cars & Concepts. I believe they were located in Brighton, MI.

Roush, Saleen, Shelby, Vector and a bunch of others might also be places to start. They do a lot of contract work with the car companies. If your idea is that unique you might want to have an attorney draw up a NDA before you go talking to companies. In many cases, they may require one to protect themselves.
Good luck
Denny