When I ran speakers in my car I used the old wires by taping the new wires to them and when I pulled the old wires out they pulled the new ones through. Be very gentle otherwise they will pull off in the rubber thing which will ruin your day. But, these were not component [because I only wanted worry about running a few wires]. If I was to mount a cross over it would be in the door itself. My buddy did this on his wrx but it took some time and some fabbing to get it to work. You will have to look at the clearance to find the most advantagous place to mount it. Good luck
