Chances are there rear brake light switch is adjusted so it is on all the time and the brake light filament on the right side bulb is burned out.
If the right side comes on when you apply the brakes and the left is on all the time, some electrical work was done wrongly. W/B is brake light hot, B/W is ground and Gray is tail light hot.
Pat- 2007 DL650A was ridden to all 48 contiguous states. 2012 DL650A is just getting started.
Nicknames I use to lessen typing, Vee = 2002-2012 DL1000s. Vee2=2014+ DL1000s. Wee = 2004-2011 DL650s. Glee = 2012+ DL650s