Great, appreciate the help, will try it tomorrowI used the instructions from the link in this post- https://golfmk7.com/forums/index.php?threads/vland-mk7-5-sequential-taillight.386591/post-7685117 (post #14)
Connecting that included yellow wire without any coding didn't do anything for me. I then tried some coding, which got things working, but not properly. Then I tried a harness from Amazon, and that just messed things up. Then I followed the instructions on the linked post, and everything worked perfectly.
So you don't need any additional harness, just follow the directions linked in the post I copied above about repinning wires, and then do the coding, and everything should work properly. At least it did for my 2017 GTI sport (USA).
Note- be sure to write down any differences in your stock coding so you can change it back if you need to later.