There are plenty of companys' parts you can put in a Ford car. Mazda parts fit many of the engines, and some parts, particularly those that have a limited lifespan, such as brake pads, clutches, shocks, struts, tires, and filters are made by many companies. There are also companies that make after market parts for a variety of makes of cars. Ford can do nothing to you for deciding to use Goodyear Tires instead of Firestone/Bridgestone, even though one is a company they own and the other isn't. Admittedly only your "Ford Dealer" (tm) can give you "Genuine Ford Service" (tm) but any mechanic can rebuild your engine, replace your muffler, change your brakes, change your spark plugs, even change the engine, and there isn't a blessed thing Ford can do, except not honor the warranty for damage caused by such replacement. For some forms of routine maintenance such as oil and filter changes they can't even do that.