2006 Audi A3 (8P Sportback) Trailer Lighting Converter Installation

The Audi A3 is part of a new generation in automotive electronics, which brings both a new ability to customize it easily as well as new issues to take into account when tapping into or modifying the electrical system. With modern technology, everything in the A3 is handled through one of several onboard computer systems. Every switch and device (including light bulbs) is part of the Controller Area Network (CAN) which runs throughout the car. This means that many features can be modified by altering the settings of the CAN system computer. Changing these settings can be accomplished with a commercially-available software program and interface cable, and is something frequently done by Audi enthusiasts.

These instructions will guide you through installing a trailer lighting interface on the Audi A3's variable-voltage lighting system in a problem-free manner. No computer settings will need to be changed, and following these instructions ensures that if any of the settings are changed at a later date, the trailer interface will be unaffected.

Here's a comparison of "the old way" vs. how the A3’s lighting system behaves:
The old way: There were separate bulbs (or separate filaments within a single bulb) for 'tail' and 'brake' lights. The difference in brightness between 'tail' and 'brake' was due to the wattage of the bulbs; they both received about 12 volts; and they were hard-wired to perform only whichever function they were intended for.

How the A3 works: In addition to dedicated turn signals and backup (reverse) lights, the Audi A3 has three bulbs on each side that can act as tail and/or brake and/or 'rear fog' lights. These bulbs are all of the same type, and only have one filament each. The computer applies reduced voltage to one or more bulbs on each side, making them glow at 24% of their maximum brightness to act as tail lights. Which bulbs are activated, and the percentage of brightness, are dependent on the settings in the car's computer. When you step on the brake pedal, the computer increases the tail light voltage to make the bulbs glow at 100% of their maximum brightness to act as brake lights. Again, which of the bulbs are affected is dependent on the computer settings. The behaviour can be changed by altering the computer settings, no re-wiring needed. A3's sold in other countries have the taillights configured differently to meet local regulations as necessary.

Unfortunately, this variable-voltage system does not work properly for US trailer wiring - trailer taillights expect to receive about 12 volts, not 3.5 volts - and the fact that the 'brake' and 'tail' lights on the car can be re-arranged in software means that those wires cannot be relied upon to always behave in the same way. There is a simple solution to this: Use things on the car that will always behave as desired. This means using the center, high-mount brake light on the car for ‘brake’, and using the license-plate lights for ‘tail’. Both of these circuits only have 'off' and 'on' states (not variable-voltage), and both only have one purpose and cannot be re-programmed. The converter specified puts no load on the lighting circuits, they are only used for sensing the voltage present.

source