Left rear turn signal has stopped working. on 2006 Mercury Mariner

I have replaced the bulb, relay, and fuse, the problem is still not fixed, left turn signal in front blinks rapidly.
What are other possible causes to this problem?

Asked by for the 2006 Mercury Mariner
The rapid flashing is being caused by the inoperative bulb. The only possible causes are a faulty bulb, faulty power supply, faulty ground or a faulty connection inside the socket. With a test light check for power the turn signal socket, then hook the test light to the positive side of the battery and make sure it lights up when you touch it to ground. If those are good, and I suspect they will be, then replace the light bulb socket.