Shortly after liftoff a passenger in row 18 reported observing flames coming from the overhead area on the right side of the airplane. The flight attendant suppressed the reported area with a Halon fire extinguisher and reported the incident to the flight deck. However, the flight attendant did not report seeing flames. An emergency landing was made at the departure airport without further incident. Examination of the overhead area around row 18 revealed that the cannon plug connector adjacent to the light assembly had shorted. Subsequent examinations of the connector revealed that pins in the mating halves of the connector were damaged, and appeared to have occurred when a previous connection was attempted. Research efforts to duplicate the connector failure mode failed. A review of the operator's maintenance records failed to disclose if any maintenance had been conducted in the area of the connector. However, the operator could not verify that the connector and affected areas were never serviced during a previous stopover.