Ok, after being busy for, like ages, I decided to upload the pictures of how I installed the sub and tapped into the speaker lines.
First of all, this is a little different, and the inside of my little beast is rather dirty. (Yes, I live out in the bush and haven't had time to clean the inside, yet)
First pic, remove the plastic trim from the drivers side lower panel. (Pull directly up. Easy)

Second picture, remove the plastic panel rear drivers side passenger side lower panel. (Again, pull directly up. Easy)

Next, pull the plastic liner that covers the pillar behind the drivers seat belt. (Pulls directly towards the inside of the vehicle)
WARNING WARNING WARNING WARNING WARNING WARNING ANYTHING that has bright
YELLOW tape wrapped around it, or is a bright
YELLOW electrical cable. DO NOT touch or cut! These are for the SRS system (Air bag sensors and seat belt pre-tensioners)
Ok, with the warning out of the way, find the twisted pair of electrical cables. (On the diesel, drivers side, rear door they will be colour coded red and blue. Red being +, blue being -)

I tapped into it with simple crimp terminals.

Picture of the speaker in the rear door (For those who are interested)

Next picture shows the strapping I used for securing the 12" sub box into the back of the i30, without needing to drill, cut or modify anything. (Also allows for a quick, easy and undetectable removal.) Cost $4.99

12" Kenwood sub and box.

Tie down points used. (Two child seat anchorage points were used.)

Drivers side secured.

Passenger side secured. (Note ratchet for tightening strap and removing)

Overview of the securing strap and sub.

The strap actually works very well. Once in, I can not move the sub, no matter what until I release the ratchet.
Next, just install an amp the normal way and use a high level to low level converter (Speaker to line level converter) The amp I used (Pioneer) monoblock already has line level AND speaker inputs.
Enjoy.
