We have a trailer wiring harness for your 2015 Volkswagen Golf Sportwagen but The Curt T-Connector Vehicle Wiring Harness with 4-Pole Flat Trailer Connector # C56391 will not work for your vehicle.
The wiring harness that we have that is confirmed for your vehicle is The ZCI Circuit Protected Vehicle Wiring Harness w/ 4-Pole Flat Trailer Connector and Installation Kit # 119250KIT. This kit bypasses factory wiring and involves no cutting, splicing, or plugging into the vehicle's wiring.... view full answer...
The part number you reference is to an older Thule Big Mouth bike rack which has been discontinued, however I believe the part you're referring to and needing is the Replacement Wheel Tray for Thule Big Mouth Roof Mounted Bike Carrier # 853-5423-56, however the upright bike arm has been discontinued with the rack and is not offered for a replacement.
If your current upright is non-operational then I'd just recommend replacing the entire rack with the Thule ProRide XT Roof Bike Rack -... view full answer...
Unless the bike has a step through frame like some women's bikes have, you won't need a frame adapter for the he Sport Rider SE # HLY74FR you looked at. I took a look that the Tern GSD online, and from the photos, the bike retention hook would grab the frame well before it bottoms out. view full answer...