I had a similar issue, I replaced the complete wire harness and solved my problem. I had turn signal/brake light issues with my 69 C10 Truck, and I replaced the complete wire harness on that as well and it solved the problem for me. It takes about 2 days to do a complete harness if you are up for the task, it is not very difficult, I am not an electrical engineer.

Hope this info helps or look at an other point of view.