Need some input on electrical problem

[okiedokie]

Doing some updates on my brothers 49 F1 that he recently acquired. It is a fairly nicely refurbished truck with the original 6 volt system. I discovered that the brake lights don't work. He is sure that they did work. I have tested the circuit including the brake light switch and it is good to the point where the harness connects to the tail light bucket. The tail lights both work, but the brake lights do not. I substituted a new bucket with a known good ground attached and the result is the same, no brake lights. Any ideas as to what I am missing?