not sure on your exact setup what's happening, but the usual hookup is that the brake light power goes from the brake switch to the turn signals - if it's just a stop, the power goes right thru to the taillights, but if the turn indicator is on, the brake signal is interrupted, or if the brakes aren't on, the signal mechanism puts the power to the brake wire ... in the light, the same element is brake and turn, if you originally had no signals, maybe when they were added, the turn indicator lights were seperate, either a 2nd light assembly or a 2nd bulb in the same housing - might be why you have 4 wires back there?
Bill
