I bought this one for my Touareg, should work for Q7 as well:

Curt Trailer Brake Controller Harness 2007 To 2017 Audi Q7 x 51522

 

sharing on this older thread, per chance it helps someone…since this thread really helped me find my brake control harness on a 2015 q7 tdi. The advice about the front plug being wrapped in gray foam was accurate for me. My harness had been bundled up with yellow tape and stuff up right behind the fuse box on the drivers side. Invisible from below, even with the kick panel removed under the steering wheeel.

1. When I removed the left side cover to the fuse panel (where you can check your fuses), it was visible on the left of the fuses. It was a bear to get loosened up though, because the larger harnesses were bound to the front dash structure with some sort of special fasteners. I didn’t want to cut them so I just pried them out of the way enough for the connector to slip down towards the floor.

2. there’s a link to an Audi service bulletin somewhere on this thread that shows instructions for Audi techs to make this cable available for customers who are installing brake controllers. It was consistent in describing the front drivers seat area with what I just experienced, but it also mentioned that the harness, as installed, was NOT connected all the way to the rear trailer plug. The harness goes to the right rear tire well area and must be connected to the trailer towing control box. The harness ends in a female plug, dangling in that well, and the towing control box has a loose male plug, dangling as well. They are, from the factory, NOT connected, per this service bulletin. So, after tearing apart my right rear paneling and determining they meant the tire well, I found the connecotrs, and for reasons unknown, they were connected!

hope this ramble helps someone, some day.