the plug that is marked speaker (that label is next to the plugs)
is the one you want to tap into ideally you would want to solder and tape them up if not use crimps or terminal block.... (never a fan of terminal block personally)
you will see on your line adaptor there are 5 wires...... they should be marked
and just match them up to the correct speaker output wires
(ignore the earth right left label inserted on the above pic)
And when you pull your Live feed wire in for your sub do it Properly and fuse as close to the battery as you can (big cause of car fires is bad wiring especially high power carrying amp cables)
you connect it to ground... nearest one i can think of off the top of my head is the black wire on the back of the fag lighter (unless you have a multimeter and can go checking)