I used a straightened coat hanger with the cable taped flush to it to get through the firewall boot. Once you pull the end of the cable through, you can untape it and pull the coat hanger back out the direction it came in. I last did this a couple of year ago to run dedicated power to a tail light converter so I can tow with my Sonic (the box uses the car wiring only for signaling and powers the trailer lights through it's own circuit). Newer vehicles are sealed much better than they used to be - I never had to go through this on my older ones.
