[patrickt]

If: 1) All four running lights work normally; and 2) All four turn signals work normally; and 3) The rear brake lights work normally, but the rear signal light will not blink when the brake pedal is also being pushed, then I would suspect the trailer light relay has failed. It is fastened to the steering column under the two quick blow fuses. It has three input wires and two output wires. The three inputs are R Turn Signal, L Turn Signal and Brake Light. The outputs are to the two rear high intensity filaments on the bulbs (one left, the other right).