Subaru has this handy rubber grommet ready to go for you.
Here's from my car when I ran my 4-gauge, inside the engine compartment. It's located near the top of the firewall area, to the left of the wiper fluid fill cup. It's fairly high in the dash. I'd recommend removing that plastic rectangular-oval, cutting a hole in it just slightly smaller than your cable, feeding your cable through from the engine compartment, and with any luck it will fall to the floor inside your cabin near the driver's side pedals...
It's from my worklog:
http://legacygt.com/forums/showthrea...ns-193295.html