I can't remember if the 68 and 69 tail lights are the same and where they ground but have dealt many light issues on these things i can't tell you how many times the issue was a bad ground or a bad connection. Get a wiring diagram. Cleaning things up and check for power along the way. Things just get corroded and loose. I know that's a simple answer that you already knew but it's usually the solution. Switches do go bad too.