Brake light out

I have a brake light out, left rear. I read your guide on brake lights and how to fix them. Excellent job by the way. Unfortunately I'm a bit lost to my problem. The bulb isn't burnt out, the brake switch is good as the other light goes on, the light bulb itself turns on when I turn on the full light system (I know, different wire but means the socket isn't or shouldn't be bad). When I attempted to test the power with a test light, nothing was showing as working from any of the sockets even though I know the turn signal, reverse lights, hazards, etc, all work out of the same junction. Tested the test light and it was working for both car battery and a 9 volts. Any thoughts?