I have an all-in-one subwoofer/amp box that I had in my prior car that I would like to add to my recently acquired 2003 S60 with the HU-613. I don't want to change any of the factory speakers/amp (for now), I would just like to add the sub box/amp unit. The three wires I need to connect are the audio line-in/RCA cables, battery power cable and ignition power cable. I plan on installing this with my friend who has decent experience with wiring car audio on domestic cars, (Chevys/Pontiacs/etc) but I know Volvos are unlike most domestic cars in a myriad of ways!! Being as I'm not looking to change the head unit, we were thinking that we could simply splice the rear speaker cable and add the line-in that way? Anyone have experience with this? IF this would work I can control the bass level with the external bass-level ****. If I plan to go this route, what would be the best/easiest way to attach the ignition power wire as well as connect the amp to the battery? I am fairly new to this so any tips, thoughts or advice will be IMMENSELY helpful!

Thank you Volvo audio experts!!!