Ok... So you haven't resorted to emailing Ford directly. I would do that next. Be sure to include any customer service numbers that you already have. Make it clear that you want this issue troubleshot and corrected and that waiting for the dealership to come across another car with the exact same issue is not an acceptable answer nor is it a resolution that you're having with Brand Spankin' New Mustang that you just dropped a metric ton of money on.
If they don't like it, then you'd be happy to discuss returning the car to Ford or exchanging it for one they need not troubleshoot.
Feel free to copy past the smileys to your letter.