On my first SG ('09), I used a detachable harness that plugged into the main harness under the seat. Simple plug and play. Google "image" tour pak harness or something similar to see the diagram for the plug up location for your year.
Sadly, that same tour pak, with upgraded LED lighting, won't plug into my current '15 SG harness.... will have to figure out the wiring pin locations and straight wire tour pak to bike.... yuck!