Yes, that is a very good idea to have a global oversight agency of the car industry. With all the cars being recalled to to manufacturing issues, it would be only right to have someone ensure the safety of these cars. It will prevent a lot of accidents, deaths, and lawsuits, and everyone would be better because of it.
A car is a deathtrap when the manufacturer is negligent, much less criminal. An oversight agency, at least intranationally is a necessary entity to protect consumers. Publicity used to be enough, but in the last decades, shame has not been enough to force ethical and safe corporate behavior. If safety standards are not kept at near equal levels across the globe, then eventually an international oversight agency will be necessary, if not popular.
In the car industry there are so many standards that must be met before a car can be sold. Why not make someone in charge of the industry to control safety features that will help everyone? Above all else with vehicles, you want to know that in the unfortunate event of an accident, you are as safe as possible. A person doesn't want to be concerned with if their car will even be able to do what it is supposed to do to protect you.
I feel that it should be handled by ELECTED national legislatures not by a global governing body. Each nation has a unique culture and national governments are closer to the people than the UN, World Bank, IMF, etc. There is too much of a push towards global controls. National sovereignty is the right of the people to control their nation's destiny.
When there is a large governing body for an organization or product, it usually ends making the product work less efficient. A bureaucratic force in control typically ruins a good organization or product. For example, the US Food and Drug Administration. It is completely ruined by our government overseeing it. There is too much paperwork and far too many people involved to make that organization work well and for it to do its job correctly. The same would happen for the car industry. Nothing would improve, except maybe for the interim, but ultimately, problems would still persist and only continue to get worse.