1157 is the bulb #. you can get them pretty much anywhere, autoparts stores even 7-11 & AM/PM. Take the red lens off, reach your fingers in the tail light housing, grab the bulb push it in a little, twist and it will come out. the bulb is a dual filiment, one is the running light the other is the brake light. your running light filament is burned out.(what oh9bolt said ^^^)