Why is one side of the Audi C6 rear taillight not working?

[DeMichael]

Audi C6 rear taillight not working on one side is due to a circuit board failure in the taillight. Types of taillights: Rear lights generally consist of rear position lights, reverse lights, brake lights, rear fog lights, turn signals, and reflectors. Functions of taillights: Position lights function to allow others to judge the width of your vehicle from a distance at night. They illuminate when the position lights or headlights are turned on. When braking, they light up to alert vehicles behind and prevent rear-end collisions. If the position lights (also red) are already on, braking will significantly increase their brightness for the same alerting purpose (these bulbs have dual filaments with two circuits). Before turning, changing lanes, or starting, relevant turn signals should be activated in advance to alert others, causing the left or right yellow lights to flash (some vehicles also have these at the front, sides, or mirrors). In emergencies such as towing, being towed, or breakdowns, the hazard lights should be activated, making both yellow lights flash simultaneously (hence "double flashing"). Additionally, some vehicles have rear fog lights, which are designed in yellow for better light penetration.