Okey dokey. First off take your airbox out (or move your airfilter). Look at your engine bay towards the passenger side strut and you will see a Giant rubber gromit thingy. Pull it out (if you can...beware its an ass to get back in) put a small slit in it and push your cable through.
Arrow shows where the grommity thing is. (its behind my blue fuel regulator, you may or may not have one)
Close up so you can get a better view (hopefully)
It should come out behind the glovebox right at the back of the footwell. Its a pig but if you keep pushing the cable through from the engine compartment you will see it come through.
Ok now remove the plastic sill cover and the other plastic bit. The parts I have marked as blue. They just pull off so dont go looking for any screws
(i dont have a picture as i was hungry and it is windy and cold) Then run your cable under it. Follow the red line i have crudely drawn on. You will have to move the carpet to one side
Then slip the cable under the back seat. Keep it under the carpet if you can. I have circled my cable in blue just so you can see roughly where I have put mine.
Then it should come out somewhere in the boot under the carpet. Nearish my red arrow. My blue line shows where I ran my cable under the carpet into my amp.