09 bmw 528i adding amp and sub

Ive added a sub and amp to a 09 528i and I get very low output, tapped in to the rear 4" mids for the amp source. my guess is that this mid is crossed over HPF. so I'm not getting enough low frequency signal to my amp. this system is odd because it doesn't have the under seat subs. does anybody know if I can take the signal from somewhere else such as the dash ( head unit ).

I don't have the link saved but if you search on google its easy to find the PDF from Bavarian Motor Werks audio department for installing aftermarket subwoofers. Look on M3 M5 forums too. Only the factory amp sub out will give you the proper range needed. That applies to almost every BMW made in the last 10 years. Definitely going to have to cut the wires to tap into them. They are twisted super tightly together. Heaven forbid if 2 wires rub together! lolTo master and control electricity is perfection. *evil laugh*

couldn't find it , sorry.