After market ampifier, where to take speaker input?

[Brissie]

I had looked at links provided by JMH1983 and other threads from caraudio section, but they mostly applied to the Us model which didn't have our base car stereo that I have.
I have a problem with the car not allowing me to access AUX/Audio and USB as montioned in my other thread. Just took the car to the dealer. Was told it was because I have interrupted the signals going back to the H/U because I have taken the signals from the spaker cables going to the underseat woofers. There are 4 wires to the underseat woofer (only two wires in US model cars as shown by photos posted by US members), two going into the woofers, and two returning signals to the headunit. I think I have cut them, so I will try to just splicing them and see how I go.
So to those who have installed aftermarket amplifiers, where do you get your speaker inputs for the amps from? And how did you do it? My amps take spealer level input so I don't need RCA.
Thanks