Trailer hitches are not typically sold with the wiring harness because not everyone who buys a hitch is using it for towing so we make them available separately.
For your 2015 Volkswagen GTI you can use Tekonsha wiring harness # 119250KIT. view full answer...
The Curt hitch part # C11000 that you referenced comes with the needed bolt leader. If you needed a replacement or a backup you would want the 1/2 inch bolt leader part # 80101-1. view full answer...