Question about installing system in Mach 460

I recently (last week) found my mustang. I had a 1992 Mustang LX with the big 2.3L 4 banger when I was 16, then I moved to a 98 Chevy Malibu, next was a 1999 Jeep Wrangler, and now im in a 2003 Mustang GT that I plan on driving for a long time. I got a really good deal at the dealer and they gave me 2 grand above trade in value on my jeep so I had to drive the mustang home. Well after looking at the 6 disc in dash changer, I realized that I do want to keep it in the dash... However, I do want a little more bass then what the Mach 460 system is giving me. I am very familiar with installing systems in vehicles, but I have never delt with this mach 460. If I add a line out converter from high level to low levels off the back speakers will it give me a good enough feed? I have heard that I need to go behind the deck before it goes to the amp and splice off those, but like I have no idea which would be better? Any input on this would help, I plan on doing this tomorrow on my day off, so the faster the better! Thanks guys...

Oh and on another note, I cant wait to get out to the track and see what I can run, It feels like I am flying after riding in a boxy jeep for a year and half!