Aux brake light
For a flashing rear brake light it's hard to beat the BMW kit IMHO. I put one on my 2012 GSA, and with the BMW harness, which truly is plug and play, is took about 20 minutes start to finish. The only issue may be availability. When I tried to buy the light from my local dealer they said it was "discontinued". Got if from Sierra BMW though without problem and they still show them as available online.
"Who endures conquers all"