Had to have the tail light on my 2014 sportster custom replaced today under warranty due to it cutting out. If you have one check to see if the lamp cuts out
I noticed it while it war warming up on my driveway the other night. I was standing behind it and saw that while the license plate light would stay lit the brake/running lamp would intermittently flick off and on. They don't have a service bulletin for it but it wasn't much of a fight with them either leading me to believe they have seen it before