There is a thread on this forum somewhere.
I used the info in the thread to split the factory amp into 4 channels to run with an aftermarket head unit.
I ran the fronts to the amp that goes to the doors and ran the rears to the amp that I separated the wiring to the rear subs.
This allows me to use the fader control to adjust the sub volume separately.
Also the stock amp is set up for high input in. Do not use preouts to run to the factory amp. 2 volts just won't give enough volume.