Here is the thing

Please listen to the Old Cat. You don't have a case. Period.

There are certain ways to resind a contract. Bear in mind, unilateral mistake is never one of them. If both you and the dealer made the mistake then you can say there is no meeting in mind and rescind the contract by mutual mistake.

You kept mentioning tricks, implying the dealer misrepresented a material fact or defraud. It is possible and may well have happened in your case. The problem is, how can you prove? You signed the contract. A reasonably prudent person would have read the contract clause by clause before signing. It is not impossible to prove misrepresentation or fraud but these are intentional wrongs so that you need to prove the intent. That is, the dealer intentionally defrauded you or tricked you. Can you prove that by direct or even circumstantial evidence?

Do not think Texas Lemon law will help because that only deals with warranty.

Do not think your attorney general or district attorney will help because the government is smarter enough not to interfere with private contract, or otherwise the interstate commerce will not even in existence.

And that first Amendment one? Don't be ridiculous. You cannot just write a letter to inform others about something the dealer did. That is libel. The dealer can sue you for defamation, wrongful interference with business, and product disparagement. Being a private figure and a private concern, most courts will not even ask the dealer to prove malice. If the dealer can show that your letter cost them 10 car sales, with damage amounting to $500000, and the court ruled against you, are you willing to take that loss so that your life will be permanently ruined? Not to mention there could well be 3-7 times of the above figure for punitive damage, if your malice if proved. And all of these for what? $3000?

I know $3000 is your hard earned money but the law is for justice, not for sympathy.