Did you guys wire the preamp from head unit direct to the stock amplifier (under passenger seat) or wire the head unit's preamp to the 3 connectors that used plug into stock pioneer head unit?
I have few pictures here inside my dashboard. Not sure which way is more easier cus I want retain original stuff (wires, amplifier and speakers) for reversing back at any time.

3 plugs to stock pioneer car stereo


A little bit lower near gear shift, I don't know what are they used for.

 

Even if the amp is only used for the subs? I may dish out the extra money for the nicer dash kit when it comes time to make a purchase.