Troubleshooting Install of Trailer Wiring on 2023 Audi Q5

Updated 03/10/2026 | Published 02/23/2026

Question:

Hi, I have a 2023 Audi Q5 Premium. Spent last Saturday tracing the wires. I able to identify the left and right turn signal wire but spent hours looking for the taillight. I ended up looking at the plug on taillight assembly on the hatch 4 wires goes in the plug see picture of left lights. The leftandright turn signal matches - Black with blue stripe and Grey with Yellow stripes. Again focusing on the left plug that leaves me with yellow, brown and blue with black stripes. Tested the blue with black stripes it is always hot. Measured the voltage it is 15vdc constantly as soon as the car is on does not matter if the headlights is on or off. The yellow every time I touch it, the car computer throw an error for the light. The brown I assume is ground and never tested it especially when it is a thicker gage wire. I attached the picture of the bundle of wires taped separately from the harness by the factory that matches the wires going to both the lights on the hatch. I have to assume the taillights is one of these wires but tested every one with the tester pen and none corresponded to the taillights. I am out of ideas. Any help is appreciated. P/S: I can send more pictures the system only allows me to upload one picture.

asked by: JAMES W

Expert Reply:

What is the error code for the yellow wire when you tested it? Did it have anything to do with the running lights or taillight circuit?

expert reply by: Jameson C

James W.

2/26/2026

It says rear lights malfunction. Not very specific or has a code.

James W.

3/10/2026

Jameson, after doing more reading I THINk the car has multiplexing function, when I touch the yellow wire with the test pen which I assume is the tail light wire. It senses an increase load and throw a bulb-out code on the car computer. If this is true. Will this kit I bought from etrailer compatible with cars that has multiplexing function or I need the no-splice type kit?

Jameson C.

3/10/2026

@JamesW The info we have for your vehicle says that it is a combined taillight wiring kit which the part # C59236 is designed to work with. Where did you see it has multiplexed wiring? If that's case that could be what is going wrong. The bulb out error code is not for sure an indicator of this but what it means is that your taillight circuit can't handle any load on it and thats why this kit draws it's power from the battery directly.