Have a towbar with 13 pin electric kit. Its a dedicated kit, not bypass, which wasnt cheap, but its the 2nd S5 I've had with same set up (Bosal detachable) fitted and coded by a local specialist. I've been more than happy with it. Disables sensors on reversing etc. plus its impossible to see the cut out unless your on your hands and knees looking upwards from the floor.
Cost was around £600 fitted
Pretty sure there is fitting instructions on the Bosal website too if required