Heres where I ran mine...
I looked for about 30 minutes and couldnt find anywhere to run it,so I drilled a hole through the firewall just to the right of the brake booster(looking at the car).I did it from inside and took a chance at hitting the booster but didnt.It is about 3 inches from it.I then cut away the insulation and continued to drill the hole bigger,I am using 4 gauge wire so you know what thats about.I then put a hard plastic grommet in the hole and ran the wire through.Once I was done with the install I went back and put some silicon around the hole to prevent water from coming in.If you have a Dremel,that makes life so much easier.
Just a thought,
Chris