Doug,
I'm not an electronics wizard, but I like to tinker and have a friend that's spent his life building and repairing anything electronic.
He made up a small circuit board to help with controling the current for the dew heaters I have. The wiring and and mounting of same was my job
If you just want to power the fan it's even simpler.
With the wiring coming from the battery make sure that it runs through a fuse (glass, small 205 type) this will prevent fan meltdown incase of a short circuit, apart from that the wiring is simple.
