Products (parts or apparel) and bodies must be licensed when they are seen by the licensor ( Ford in our case) as representing a trademarked design or logo. In the case of the bodies they represent a “trade dress” design that the licensor claims is their property. In these cases, the licensee is obligated to “officially license” the item before it is offered for sale.
Trade dress simply means if it looks like a Ford, then it must be licensed. If we build a body without any skins (a skeleton), and it does not look like a 1965-'70 Mustang, it does not have to be licensed. The moment features are added that begin to suggest the overall body design (quarter panels, top skin), it must be licensed. Some manufacturers will voluntarily license a product even though it does not require a license just so the item can carry the “officially licensed” logo.