Ah, rear signals, wonder why I always thought it was the front ones? Rears are even easier especially if you put them in your topcase. I just put the Admore kit in my Givi 470 but they also make a plain strip you can put on either your topcase or along the back. I've seen several people who have upgraded to it. It has really improved the visibility of the brake and turn signals on my scoots.
I think you are on the right track to check for a lose bracket or something rubbing, hope you can track it down.