Seems like the last time I looked at the optional equipment book there was not a factory sub for the S80. And from what I have seen and heard it is not easy (if indeed possible at all) to add an amp and sub, at least using any sort of pre-amp signal. I think about the best you can do is to use a speaker-level input from the rear deck speakers to your amp. The other problem is in getting a turn-on signal to the amp. And finally, the VADIS OBD system is pretty picky about ANY sort of electronic device added anywhere to the car!