The flasher relay is part of the relay cluster of a car. The location of the relay cluster varies depending on the make, model and year of the car, and the user's manual shows its location.
For example, the flasher relay in a Ford Focus is part of the relay cluster located either underneath the driver's side wheel well or the underside of the steering column. Within the cluster, the flasher relay for the Focus is on the right side two or three plugs down from the top, depending on the model year.
To replace the flasher relay, A car owner should expose the relay cluster and then firmly grasp the flasher relay plug and remove it. He can then insert the new flasher relay, which only fits into the plug one way.