I suggest you get yourself a MTX Re-q if gets wired in right before your headunit and you just plug the two wires needed for a subwoofer. You can get the Re-q from amazon. I put the regular Re-q in my friends 2010 mazda 3 it works out amazing. with his stock headunit.
Heres a video.
2004/2005 Ford Focus Saleen
The cops are here to keep us safe.............I'm here to keep them busy!
Hatch Nation #43