We have a 1998 Buick Regal LS. When siganling for a left turn the rear signal light will not work (get that rapid flashing thing--front light works). All the emergency lights flash. If we pull the turn signal lever down to the left turn position (while the flashers are going), then turn the flashers off then the rear left turn signal works fine. Have tried replacing the bulb, the socket, and the battery. No go. What to do?