Unless it's a small dealership, I doubt that a principal signs each and every sales contract. Legally, in an agency relationship, the principal is the person who gives authority to another, called an agent, to act on his or her behalf. My point is that, by making the warranty commitment part of the deal, those provisions should be as enforceable as any other portion of the sales agreement signed by you and an authorized agent of the dealership (whomever that might be), but ultimately that would be up to the judiciary to decide. Either way, it's better than having only a salesman's verbal commitment.
Rusty