Thanks everyone for your help! It did turn out to be the whole turn signal assy. I guess after 42 years it's bound to go bad. I tested the new one before removing the old one and it works fine now just have to install.
I'm having the same problem w/ my '70, all turn, running and hazard lights are working but I have no brake lights. I just replaced the brake light switch and still no brake lights. I checked voltage at the brake light switch and there is something there but not 12V, something less that 1V. Put a jumper wire across the switch with no change, still no brake lights. The diagrams that I have for the '70 show 12V coming via a green/red wire through a 15A fuse (also goes to the hazards which are working) to the stop light switch. A green wire goes from the switch to the turn signal switch then to the lights. Does that sound right?
If my hazards work and the wiring diagram is correct then the only reason to not have 12V to the brake light switch, I would think, should be a broken or shorted wire. This may be why I show some voltage but not 12V. I'll check the current to the hazard flasher to be sure.
OK, I'm an idiot! I DO have 12V at the brake light switch. I'm guessing that that would indicate the turn signal switch. My switch is definitely on the worn side. Does not cancel for right and does not stay engaged (have to hold the switch in the on position) for the left. Unless someone has some other ideas, I'll replace the switch or rebuild it. A new switch runs $70 from Mustangs Unlimited and a cam repair kit is only $10. It sounds like replacing the cam switch is easier as well as cheaper but will it fix the cancel/lock problem?