Are the backup lights and license plate on a 2002 Buick Century wired in series?

No, the backup lights and license plate lights on a 2002 Buick Century (or any modern car) are not wired in series. They are wired in parallel.

A series circuit means if one light fails, they all fail. A parallel circuit means each light has its own path to the power source, so one failing doesn't affect the others. Since it's highly undesirable for a single bulb failure to disable all the lights, they are always wired in parallel.