When we bought our '71 I noticed that the turn signals fired both rear indicators together. Figured it was a ground problem and marked it for follow up.
Since then I have cleaned all of the front grounds, replaced the body ground/negative battery cable, tried manually grounding the taillight housings, checked the bulbs, isolated the headlight and turn signal switches and still no change. While checking the brake light switch, I can see it has been replaced and the harness spliced to accept a later style connector, but I saw no issues.
When you turn the signals on, the dash indicator and front indicator work correctly, blinking left or right. Both of the rears blink together. When you press the brake, whatever lights were blinking (either side in the front and both rears) light up solid and the signal flasher stops. I've gone over the wiring diagram and I'm not sure what else to do besides start peeling harness tape off and looking for fried wiring. Is there a connector between the dash and the rear of the car I can isolate somewhere? Any advice appreciated.