First check that you are using the right fuse, the proper amperage rating will be listed on the fusebox. Never use a higher rating than Ford specifies, the car can catch fire. If that's not it, you have a short circuit. Look at the wires at the taillights. Look for corrosion, loose connections or missing insulation on the wires back there. if that doesn't get it than look for flooded taillight assembles or bad lamps. If still no good than you're going to have to trace down the taillight wires to find the problem. Again look for loose or missing connections, corrosion or missing insulation. It can be a tedious process but just keep going until you find the culprit.