Tail Light Wiring Help Needed

Hi guys,
I am a fairly new owner of a 72 Dart and I am having some issues with my tail lights. I have 3 wires that connect with the rear tail assemblies, a violet (goes back up light) and a brown and black which go to the tail/brake/turn light (1157 bulb). I get power to the brown wire when the brakes are pressed and when the blinker is on. That seems correct to me. However, when I turn the headlights on, I get power to both the black and brown (without the brake depressed or blinker on). This does not seem right to me. It seems to me that I should only be getting power to the black when the headlights lights are on. According to my wiring diagram, the black connects with the side marker lights. However, I am puzzled why I am getting power to the brown when the head lights are on. Any advice would be appreciated.