Camper shell 3rd brake light doesn’t work when headlights on.

I watched a couple YouTube videos and attempted to wire a 4 pin flat trailer plug from the 3rd brake light to the taillight . I was happy when I checked if my wiring was done correctly, when I step on the brakes all my brake lights function properly including the 3rd brake light on my camper shell. But , as soon as I turn on my parking lights or headlights, the 3rd brake light does not function . I wired the green wire

 

Try changing the grounding point.

 

When you turn on your parking lights, does the 3rd brake light come on?
OR does it not come on at all?

 

When I turn on my parking lights or my headlights , as soon as I do either, the brake light will not light up from a press of the brake pedal. When the headlights are completely off , I can step on the brakes and all of my brake lights function properly. The 3rd brake light does not illuminate at all after headlights or parking lights are turned on, whether I press the brake or not. But again, to be clear , the 3rd brake light does illuminate properly with the press of the brake pedal only when the headlights are turned completely off … I’m going to move the grounding location as suggested and see if that fixed the problem

 

The ground likely won't change anything.

What are you calling the the 3rd brake light?
The 3rd brake light is usually referred to as the on on the back of the cab up top.

I think you tapped into the wrong wire.

I have diagrams, but I need to know exactly what the 3rd brake light is......

 

I'll bet that you used the parking light wire as the ground for the 3rd brake light. The wire will show ground when the parking lights are off but goes to power when they are turned on. Hence why the 3rd brake light works when the parking lights are off but not when they are on.

 

It was the ground . Thank you

 

It was the ground , I grounded to the wrong wire . It’s all functioning properly now . Thank you