I do this for a living so, I'll jump in here and offer some ideas.
First off, DO NOT put your own money into this unless you are willing to lose it. Even a good idea is a crap shoot.
Focus on what you are trying to sell, you are trying to sell the concept right? Let's say you add "value" to the concept with CAD drawings and prototypes. Have you increased the value of the concept to the buyer because you have a CAD drawing and a prototype? Simple answer, if the potential buyer does not see the value of the product in the concept, the CAD and prototype will not change their mind.
If I were you, I would go to
www.uspto.gov and do a search. Or, probably easier is to go to Google and where it says "type of search", there is a listing for patents. Make sure you are not doing something that has been done before (and don't be discouraged because you see something similar). This is your call but, you could apply for a provisional application at he patent office. If you do all the description and illustration, this could be done (with a lawyer) for about $800. You could do it yourself for much less but it is a major hassle and, if you don't get it exactly right, you application is denied and you forfeit your fee.
The good thing about a provisional is that lots of people won't talk to you (even with an NDA) unless you have this. This protects them and you. The bad thing is, it costs money. You say this is an automotive product. If it is a product that you intend to sell to Ford or GM, you will need at least an issued patent application (international also). And, it better be a good one. Cost of a good one is $25K and up. If it is a product for the automotive aftermarket, then you may be able to get by with the provisional.
I do this for a living and, I would strongly urge you to do the following:
Educate yourself about your product and it's market potential. You might be able to do this by looking at a similar product that is in the marketplace.
Find at least 3 companies that you think could benefit from your product. Keep in mind that a product that is a new market or "incremental" business to a company is more valuable than one that takes sales away from an existing product offering.
Contact the companies by phone. A good person to contact can be found by looking up patents that were assigned to the company you are calling and finding the inventor. Ask them if they have a product submission process or they have in-house or outside legal representation. These are the people who are going to provide an NDA.
From there, you are set. Like I said before, this is like Vegas with a very low incidence of success. However, don't let me discourage you. The ones I have seen succeed are the persistent ones.
Good luck with your venture!