Brake light troubleshooting

The bottom brake lights on my 49 buick regal will not turn on, while all of the bulbs lightup when the headlights are on, and the brake light in the center of the back windshield works properly. I changed all of the bulbs and checked the fuses, but the lights are still not working, any suggestions?

Thanks!

Check for current in and out of the stop lite switch then check the circuitry through the signal switch.