I have very similar experience as yours, but mine is behind my left ear (same side I have tonsillar cancer, BTW). Two to three times a year, I would find that at the back of my left ear, a line of crusty scab of about 3/4" long near the bottom of ear. Mine was not a "rash", more like a scar from hard scratch. Usually, it would ooze some pus and streak of blood and I apply Neosporin to it, then it heal in 4 or 5 days.