I would agree with possible ground, flasher switch and turn signal switch. All three are easy to work with. Check every possible ground. Then I'd check the flasher switch. I actually rebuilt the flasher switch in my 69 Barracuda. The back was held on by 4 metal tabs, and if you bend them up really carefully, You can get to the inside of the switch. Just be careful; if I remember right (it's been a while) there are a few small springs also, but it is easy. It is a very simple setup with a few sliding contacts. Clean up the contacts up with some 600 grit, put some di-electric grease in and put it back together. If that does not do it, I'd look at replacing the turn signal cancelling cam and/or the turn signal switch.