I ran 12awg to my front doors a few weeks ago and it was a bit of a pain getting the wire into the door itself. When you take the boot (i guess that's what its called) off the door youll notice that the wires don't run through it like you'd expect but that its a plug that goes into the door. Thankfully most of the bottom plug isn't used and you can drill through it and run your wires through. Its hard to explain, and unfortunately i didn't take pics when I did it but hopefully that made some sense.
On the passenger side I think its a little different. Ill try to take pics tomorrow if I remember.