Here's how I plan on doing it, as many others have. There are a few posts here and on other forums where people have used this method, but the pictures weren't the greatest IMHO. Here are mine.
If you look under the dash, follow the hood release cable and you'll see this:
The area above the main harness there is fairly roomy, maybe even enough for a couple 0 awg. I plan on pulling out the rubber plug there and replacing it with a grommet, should be plenty big enough for the 4 awg I plan on routing through.
Here's basically what you need to do to get to the other side of those grommets/plug:
Basically follow Rallyarmors instructions
for doing the front flaps, then remove two additional clips inside the wheel well itself, and pull the inner fenderliner back. I was able to do it simply by turning the wheels as shown, but you'll need a very short stubby phillips driver to do this.
Next two pics show what's back there:
And last how you get to that area from the engine bay:
I'll post up some better/completed pics once I've actually done it. And I might even wash my car for those pics...