James:On a '57, the flasher is attached to the back side of the instrument cluster, I would think '58 would be the same. It is secured by a springy clip, just slips into the clip. The flasher may be rectangular in shape so it is not quickly recognized. Round flashers will also fit into that same clip. Don't confuse it with the instrument voltage regulator. It is fastened to the cluster by a single screw.