In the rest of the world Audi provides plug-and-play wiring for trailer lighting. Since Audi doesn't offer a hitch for the A3 in North America, they don't offer wiring either. The European wiring kits are very expensive and don't correspond to US trailer wiring conventions, so importing one was not an option. (Click here for an article about my custom hitch.)
Since the A3 has a software-configurable variable-voltage lighting system for the taillights, and most models are equipped to detect failed bulbs, a solution had to be found that would work reliably, would not interfere with the car's diagnostics, and would avoid problems associated with how the A3 taillights work. I was able to do this using a trailer wiring converter that protects the car's circuitry and puts no load on the lighting circuits. When connected to the correct circuits (not the three tail/brake/rear fog bulbs on each side) it works perfectly, and if any taillight settings are changed (using VAG-COM, for example) it will not affect the trailer interface at all. The required converter is the Hopkins Manufacturing (Hoppy) 46365. To find one, search for Hopkins 46365 or Hoppy 46365 on Amazon, eBay or Google.
I have produced step-by-step instructions to guide you through installing the trailer lighting interface on the A3 in a problem-free manner. It's about a 1 to 1.5 hour job. These instructions are also useful for anyone wanting to open up the side panels in the cargo area and/or remove the subwoofer, since they contain details and photos on how to do so.
Click the image below to download the instructions as a PDF. The download file size is about 5MB. You will need Adobe Reader to view and print the instructions.