Actually, I think I've misunderstood.
Where are the actual owners stored? In the owner table, or the registration table? Because from the last reply, I deduce that the owner table does not actually hold the owner itself (and therefore is incorrectly named).
I don't know how your local rules are, but in my country, every car has only one owner. A car cannot have no owner (unless it's fresh from the factory and not sold yet, but even then the dealer could be considered the owner). Therefore, the owner could be considered a foreign field of the car table.
Now if your owner table does not include the actual owner but the registration instead, then that should be a foreign field of the car (as a car can have only one registration), and the owner should be a foreign field of the registration (which it is if you swap table names).
Now you can also figure that each registration has one car (which is also true). This is, IMHO, a case of a 1 to 1 relation - a registration can be for only one car, and the car can have only one registration. In most cases, the tables can be merged into one, containing fields of both tables.
As for the entity table, perhaps
you should include this in the owner table. A tax number is a property of an owner (person or company), not of the car registration.