Just contact the guys (Patryk) from bimmer-tech (
http://www.bimmer-tech.net/shop/bmw-...udio-retrofits). I also thought that my subwoofers were not working, however it was a matter of the OEM amp (I had the hi-fi sound). After changing the amp, the sound became EXCELLENT and I was able to notice that there are a couple of subs below the seats.... Hope it helps