If you look very carefully (it's really hard to get to in the van series), you'll see an extra teat on the main power cable, pass-through, grommet. It is in the lower right of the firewall as you stand in front of the vehicle. It requires you to snip off the teat from the engine side. You'll need to pull up the carpet on the driver's side to get to the grommet. It is a double wall passthrough since it needs to be sealed tightly. What ever you do, don't probe it with anything metallic!
The resulting hole is large enough for a pair of #8 wires, but little else. If you use a RigRunner, the main power cable will pass through with some work. You'll have to attach the fuses afterwards.
If you want more info on installing and wiring, go to my web site and read up.