It is true that the tail lights have a two-filament bulb, but, the turn signal and the brake lights use the same filament. If either the turn signal or the brake light works, the problem is not the bulb, or any wiring issue in the trunk area. What you have is almost certainly a bad turn signal switch. If the problem was upstream from the turn signal switch, neither brake light would work correctly, because there is only one wire going from the brake light switch to the turn signal switch. If the problem was downstream from the turn signal switch, both the brake light and the turn signal would have the same problem on the same side. Since both turn signals work correctly, that means that the problem isn't down stream from the turn signal switch. That leaves the turn signal switch.