First off, just go right to the battery from there, Beemer's sure do have a way of making subwoofer installation easy
. Second, what kind of Beemer do you have?
Otherwise, My 96 318i had Green, Yellow, Brown and Blue for the rear speakers, the Brown and the Green were to the tweeters. I would suggest using the Yelow or Blue to hook your BassLink up to because they would have a full audio signal and not just the Highs from the Amp. Hope this helped!