RE: 'Fridge Fan Wiring
Paul, as a good rule of thumb simply run an independent set of wires, hot
and ground, when adding something like that.This avoids accidentally having
your new circuit find an alternative ground path through something  that you
didn't intend.  When I added an exhaust fan (replacing the external water
intake) I ran both a hot and a ground back to the fuse block I've installed
that I use for adding any new circuits off the aux battery.  I just threw a
switch in the line and mounted it in the little grill at the rear end of the
kitchen cabinet.  I've found that it is easy to use speaker wire (flexible,
two lines, one marked for polarity) for such light circuits.  And when I
added a cutout/overide switch the the fan on the back of the 'fridge itself
I just cut into the line between the thermostat switch and the fan itself,
didn't mess with any other circuits.
Low voltage (read susceptible to circuit resistance) DC circuits can get
tricky if you start trying to tap into a convenient source somewhere that
seems convenient because you never know what you are doing to the existing
circuit.  Just ask anyone that has had a stereo installed by a lazy tech
that just tapped into the first 'hot' wire he could find, a lot of horror
stories there.