srs

Hi, I have a mk1 ghia galaxy & I want to change the head unit to a sony.I've bought the iso converter & connected the power side of it...but where is the speaker connector located? I have seen a metal box under the stero hole...is this an amp?...and does the connecter from it it fit direct to the iso connector for the speakers? Hope you can help..as its getting me frustrated! Many thanks srs