You need tools to take the backs off? Just fold them down! haha

 

The rear seats just pull off. Just use a little bit of force, no tools required. Then run the wire up the side trim into the area under the seat. You should be able to feed it through the underside of the seat back. You may have to fold the seat back down a smidge to get the wire underneath it. Should be pretty easy.

 

Sorry if im being dumb here, but my rear seats dont fold down, its just solid metal behind the seats with a wire loom running up there. I tried ALOT of force to pull the seat backs off but they wont budge.
I can pop off the bottom of the rear seats no problem, but i couldnt see anywhere to run the cables. am i missing something, keep in mind im in the UK, so im running the cable up the passenger side (your drivers side)

 

I thought all A4's had folding rear seats... You don't have any levers on the outside shoulder of each side of the rear seats?

 

Nope, no levers. The back of the rear seat is bolted to the metal behind it. And the view from the trunk is the same. solid metal.
Maybe its because mine is the UK 1.9TDi version, I dont know. Its a nightmare!!!

Is there a way near the wheel arch to get in??