Thank you for your question. I look forward to working with you to provide you the information you are seeking for educational purposes only.
If you had a signed contract to this extent then mistake or not the dealer has to honor the contract. Unilateral (one sided) error does not have to void the contract. You need to tell the dealer that they will either honor the deal or you will file a complaint with the DMV Dealer Licensing Unit and also with the Attorney General's Consumer Protection Unit for investigation of their unfair and deceptive business practice.
You can enforce the contract as written and if they refuse then you can also sue them in court for specific performance on the contract, which is to enforce the contract as written.
I truly aim to please you as a customer, but please keep in mind that I do not know what you already know or don't know, or with what you need help, unless you tell me. Please consider that I am answering the question or question that is posed in your posting based upon my reading of your post and sometimes misunderstandings can occur. If I did not answer the question you thought you were asking, please respond with the specific question you wanted answered.
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Also remember, sometimes the law does not support what we want it to support, but that is not the fault of the person answering the question, so please be courteous.
This is NOT the practice of law nor is it legal advice to you, it is merely educational information for you to use to seek out a licensed attorney in your state to get actual legal advice from them. Please use sites such as http://www.martindale.com or http://www.lexmundi.com or http://www.hg.org to find a local attorney to get actual legal advice in all matters.