Yes. Classic symptoms of a failing negative ground connection.
Some older bikes had a similar problem in the front because the head bearings no longer provided a good ground to the front turn signals.
You can start at the rear turn/tail/stop light assemblies and trace out the ground.
OR you can get a piece of stranded wire and run a new ground all the way back to the battery.
You can test that first before you make the final hidden run just to be sure it fixes the problem.
Connect a wire to the (-) battery terminal and run it around the outside to the rear to the "ground" connection point on one of the lights.
