If you have non-HK, there is no other amp. The speakers are running off the head unit. You can use an RCA inverter, and tap into the rear speaker inputs.
Check out this link. It's for an HK retrofit. But it will show you the pin outs/wires for the HK subs. You can take those. Beware though they are at a cut off point for the HK subs. Probably not the best freq cutoff for aftermarket sub setup. http://www.mindspring.com/~pmbenn/hk_retrofit.htm
A forum community dedicated to BMW E46, E90, and F30 owners and enthusiasts. Come join the discussion about M performance, turbo kits, engine swaps, builds, modifications, classifieds, troubleshooting, maintenance, and more!