I'd replace the bulbs in both tail lights. There are 2 filllaments in the bulbs one for the turn signal and the other for the brake/tail lights. On fillament is burned out while the other is'nt. I'd also recommend using Sylvania Long life bulbs. I believe the correct number is 3387LL hope this helps! If that dose'nt work you may need the brake pedal switch. Try the bulbs first.