No L/H Directional Lights

On my 68 GT having somewhat of the same issue, my right rear running light works great, but no brake light or directional on right rear. I went to the connector under the kick panel and tested there, I believe it was a brown wire like your drawing shows, but it's dead there. It's weird that yours is dead front and rear though, could be at bulkhead connection there notorious for issues. Following thread curious to know what you come up with.