That's wha I would suspect is the Ground terminal. Find the point where it attaches to the engine, and the second point where it attaches to the frame.
The starter relay uses the ground circuit for the brake light so if the frame/engine grounds are bad it will use the light bulbs resistance as ground, and over heat the contacts in the relay making the arc shut.
Jus a maybe but I ould double check these points. Loosen them p, and clean the contact area to be sure.