What causes the left brake light not to light? I.

What causes the left brake light not to light? I put 2 new bulbs in. All 3 brake lights went out and I replaced the brake light switch twice this year. The pigtail on the passenger side had 1 burned socket. I couldn't find the pigtail, so I had to splice a socket into the line. There is a black wire off of the bottom socket on the right side that isn't connected to anything, but none on the left. These lights do not have the circuit board. I have all lights on the left except for the brake.