Here are some tricks on how to find those wrings inside your trunk.
In the top right corner of the trunk, you'll find this grip:
This picture is a bit "NOT CLEAR" as the wirings coming out of this grip cannot be seen. If you look carefully in your car, there will be "four" wires that are going in but aren't coming out to anything. Those are your speaker wires.
Frankly speaking, I have no idea which is positive or negative since the color coding is a bit complicated for a non electronics guy. I detected them via hit and trial.
To install the speakers, you need to get the rear seat removed. I did all of it by myself but I suggest you get it done from a mechanic since its a pain staking task and you might break a plastic panel while doing that.
Still, if you want or anybody wants, I can prepare a full guide on how to remove the back seat properly. :)