Easiest solution I think:
Have Owners table without any info of car(s);
In Owners table, along with regular owners, have a record for 'No owner';
Have Cars table, without any info for owners;
Have a CarsOwners table like tblCarsOwners: CarOwnerID, CarID, OwnerID, ValidFrom.

Any owner can have any number of cars registered in tblCarsOwners;
All cars without owner estimated are registered to be owned by 'No owner' in tblCarsOwners;


For any car, its ownership can be changed at any time moment, by adding a new record into tblCarsOwners, and determining the start of ownership entring the date into ValidFrom field (create an unique index for tblCarsOwners based on carID, OwnerID, and ValidFrom, to avoid registering the car to several owners at same date). The ownership of car at certain date can be calculated using tblCarsOwners.ValidFrom field. My advice is, create an UDF for this.

Additional bonuses with this design:
You have a history of ownership for any car;
You have a history of cars owned by any owner.