The left rear running light stopped working but brake light still works, while right rear side works fine. Swapped out right for the left and left side works on right side but right won't work on left side. Can't find any blown fuses???

Any help for this problem?
Just discovered that with the vehicle running and I turn on the headlights, the left rear running light will quickly flash three times and then stop working, almost as if it's flashing a code signal. There are no service engine light alarms.