If you crawl under the dash the factory harness is just a couple of white connectors.

The Fusion radio harness is wired to 1 of those. Follow the red and yellow wires directly from the back of the radio until they join together. Most likely this happens where it meets the white connector skiers installs. Mine did. From there you can modify the harness to the way you want it.

If you just don't want to hurt a brand new boat (I get that), the connectors can be purchased online and you can wire a new harness and just unplug the factory unit and plug your new one in. The harnesses I'm referring to has the speaker wires and power wires separated in different connectors.

If you ever want it to be "factory" again just plug the harnesses back together and remove the 1 you installed and you didn't cut any factory wires.