Check on top of your drivers side wheel. The battery cable runs under the air filter, over the wheel well, and into the fire wall. The cable is in two parts and is connected with a bolt type connector under a plastic cover. Look to see if that connection is corroded. It is in a spot that can get wet. If it is you can replace the cable ends, and just use a short bolt to make the connection. Just tape it up real good, or you can go to Ford and buy a new connector. I had the same problem.