Issue with one of the brake lights not coming on when hitting the brake paddle

jimjimjimmy said
grounds would be my first check . bad ones can cause some weird she-ot

So it was the ground. I pulled the light out of the taillight and put a ground wire from the light to the car to test it and it worked. But how should I tie it together to look nice? And why is it not grounded enough from that taillight? Thanks