B series cars dont have RCA outputs on the head unit, so using a line level converter to transform the speaker signal to RCA is the way to go, something like the audiocontrol LC2i so that it boosts the input voltage signal to match the gain level on the amp, it converts one pair of speaker inputs into two pairs of RCA's, one for bass output and another for full range amp to run your front & rears. I have one of these running a blaupunkt burning desire subwoofer amp with a JL Audio 10w6v2 sub. Im still yet to put the kenwood amp in for a pair of splits.