How can you route wires thru the firewall of a 2005 Impala you want to mount a switch inside the car for a project but cannot seem to find a route for the wires?

Routing wires depends on what you have to work with, Like due you have a basment? is there a closet under the area you need to go? Or are you asking whats the best way to go thru walls? I my self...

You dont tell us which model you are after but you could try here... http://www.main.experiencetherave.com:8080/subaru_manual_scans/ there are a few wiring diagrams.

is connected to negative pole and to body of car

By power wire, I assume that you are talking about an amp or another accessory to that sort. You will have to look up behind your brake pedal toward the corner, there should be a small hole cover...

put simply you just run a wire from the switch directly to the starter bypassing the key start as you do so.