Also have to ask have you verified the lamps are the correct ones all 4 corners and they work.
Have any wiring changes been done? Switches been replaced?
Both rear lights flashing when right turn is on makes me think switch OR messed up wiring.
Does the emergency flasher work as expected?
I had a situation where while I was replacing my left front fender ( 67 Dart) I had the left front headlight turn signal harness not grounded and the turn signals would not flash at all - goes to the good grounds thinking.
Time to download a wiring diagram and start tracing (might have to go with dart or valiant or a year newer but will help guide you)
