Ok guys, you will need to disconnect your back door speakers and run your wires from there.
I just did mine last weekend in my berlina and this was the way i did mine:
Remove the plastic strip next to your drivers seat(just lifts off with a screwdriver) and you'll find 4 spine head screws, undo these and the plastic section lifts up.
Under there is the wiring, this is where i joined mine because you can do both left and right speakers here.
Wire colors are as follows: Rear left(+)Brown/black(-)Yellow/blue.Rear right(+)Blue/orange(-)Blue/black.
Remove the whole plastic strip so you can hide your wires right to the back seat.
I take it you know you have to remove the back seat and parcel shelf to install them?
All i can say is it's well worth the effort, made a heap of difference to the sound quality!
If you need any more details or have any question please pm me and i'll be glad to help you.