Brake lights ground where?

Ok let me ask a real stupid question: on my 66 dart, I am thinking about trying the LED option on the back lights.

I noticed something interesting today, I pulled the socket out of the light body (brake) and hit the brakes at it lit up. It is my understanding that these light sockets ground via the light body, so if I remove the socket from the body, how can it still light up? Is it using the 2nd contact in the socket as a ground path somehow???? Please enlighten me. Otherwise I think I may have a bare ground somewhere?
I also noticed that one of the lights was acting weak, but when I inserted it back in the light body, I see a little mini spark and the light is full power now.?