This would be funny if is wasn't disgusting. Carrol Shelby has been bitching for 20 years about Ford's refusal to protect the Cobra name and likeness.
In my opinion, this is probably a law firm that spoke with some senior middle manager at Ford and convinced him that the company's rights were being trampled upon. Come on, $5,000-$10,000? That barely covers a law firm's cost for drafting a letter for a client like Ford, much less the costs of identifying potential "violators". That's why it's a form letter that will be sent to dozens of small businesses. I suspect that this is a firm working on speculation. In exchange for "defending" Fords "good" name without being paid a retainer, the firm gets to keep whatever settlements it receives. Any payments or court judgements recieved from targeting these small businesses would not even constitute a single drop in the bucket of Ford's current problems.
Over time, trademarked names have become commonplace. The best example of this is probably Asprin. It was originally trademarked, but that name became commonly used to refer to any pain killer. In the 90's RollerBlade fought tooth and nail to keep the term "rollerblading" from print. They did not want their specific product name lost to the public domain. "Mustang", "Thunderbird", and "Bronco" are product specific; Ford is not interested the "Thunderbirds" cartoon or the Denver Broncos. They're interested in companies that are using Ford's logos and the likenesses of its products to make money in the automotive market.
As for the registered trademark, you can search the US Patent and Trademark Office's records for filed trademarks. If you search "Bronco" you will see logo filings for products ranging from cigarettes to barbed wire to aftershave to our favorite trucks. Each filing seems to have its own logo although most logos appear to include the image of a horse and cowboy. There are several Bronco filings belonging to Ford. I imagine that they registered the Bronco script for almost every kind of distribution under the sun. BTW, it appears as though the first commercial use by Ford of the Bronco logo was on April 28, 1964. It's their trademark, they have the right to enforce it, but that enforcement cannot be arbitrary and capricious. If several companies recieve the threats, I imagine a class action counter-suit hinging on unequal enforcement of Ford's trademark rights could be possible. However, the likely result of such an action would be Ford's cracking down on everyone equally.
This situation strikes me as either another mindless bureaucrat on a power trip or a greedy law firm looking for easy money from threatening small businesses. It is much cheaper to roll over and pay $5,000-$10,000 than it is to fight this battle. It doesn't matter if the fight is with Ford or the law firm, both have deeper pockets than the businesses they're attacking.
Rather than a petition, I would suggest a letter writing campaign simply for the visual impact of piles of letters as opposed to signatures on a list. Additionally, shareholders are supposed to have the right to speak at a company's annual meeting. This would make an interesting question if the meeting hadn't already been held this year.