Old thread, but I thought I'd share how I ran my power wire. Let me preface by saying it might just be better to run through the fender, because this was a bitch and a half.
In the engine bay, just to the left of the brake booster is an empty grommet. I decided I wanted to avoid the fender run, and use that instead.
Unlike previous WRXs this one is behind two hard lines for the brake booster so its not easy to reach.
Pulled the grommet and found the sound deadening was behind it. I had to go under the dash in the driver seat and chip away at the foam. Had someone feed the wire from the engine bay.
After about 2 hours of swearing while laying on my back in the foot well, I finally got it.
I have the cable running down the center of the car now but I can move it to the side easily if I need to.
Like I said, this may just be a cautionary tale to illustrate that the fender run might be a better option to save time and sanity.