I'm betting that the voltage is at least 16, and probably nearer to 18 volts when the lights go bright. This is usually a problem in the ground side of the circuit, and often is corrosion or a bad connection at the battery or at the engine, where the big cable grounds. This results in a lot of wacky problems, usually with high-load circuits, like the alternator. The voltage regulator thinks that the battery is discharged, and tries to charge it up by going full field.