soooo... you want a sub now?
You need to run RCAs and switch wire from the headunit to the amp
You need to run power and ground from the battery to the amp
Then obviously wire from the amp to the sub
Run the RCAs and switch wire from the back of the headunit, down to the floor of the driverside door, keep the wire up high and hidden... Pull the plastic foot guard off the chassis near the carpet, through to the back seat which you will also have to take out, and then through to the boot... if that is where your amp is going...
To get the backseat out, there should be 2 10 or 12 mil bolts holding the bottom part in place, then 2 plastic clips in holes holding it really in place... Getting this seat out may be the hardest thing you ever do... it sucks...
Once that's done, you can see 2 bolts holding the back half of the seat in place, undo them, then slide it up... It should have just enough clearance to pop up and out...
THEN you will see a metal plate behind that and some fabric shit... piss that off too...
There's a lot to be done, it isn't really a small job, especially the first time you do it... what state (aus)are you in? Someone might be able to give you a hand