I'm presuming you're trying to find the power wire for the audio system?
I would refer to a wiring diagram for your vehicle. Two sources you could try are third-party factory service information providers such as AllData (sometimes jokingly called "SomeData") or use Subaru's very expensive STIS (Subaru Technical Information System) which is the same information dealership technicians use to service Subaru vehicles.
"Why would I want to profess my feelings for some 3D girl?" - Tomoya Aki from the anime Saekano