Another brake light failure help

[Corbin]

I have an issue with the main brake lights but only when the headlights are turned on both sides go dim like barely even lit I have cleaned up the ground at the rear of the truck and it didnt end up helping the issue. 3rd brake light has 0 issues

 

[thinger2]

I have an issue with the main brake lights but only when the headlights are turned on both sides go dim like barely even lit I have cleaned up the ground at the rear of the truck and it didnt end up helping the issue. 3rd brake light has 0 issues

Bad grounds
Could be anywhere but left head to firawall stud to frame or trans to frame or transfef case to frame or trans to frame or cab to frame or bed to frame
Bad ground, crusty wires.
Welcome to the sucks club.
But, fix it once, drive it for twenty.