If you are going to run a wire from the battery area, I would use a relay triggered by the aux connections on the fuse box. The aux fuse is only a 5 amp. Using a relay, you connect the actual power line to the battery and use the aux power to trigger the relay. This way you are not limited to the 5 amp fuse. Just fuse the power line separately. You can also add a fuse block to the output of the relay. This will allow you to add more accessories later.
As for fishing the wire, I have a piece of 1/4 inch polyethylene tubing as the fish tape.