It really depends on the floorplan of your unit. I have seen power swiped from the 12 volt
wall outlet, or the center roof vent in the kitchen. If you have a rear bath, there should be a wall outlet with a tv connection that could be tapped into or if it is a center bath, the 12 Volt
outlet wiht antenna is just forward of the refer.
If you come off the fan you may have to drill holes and fish wires it all depends on how you want it look.
The power draw for a circuit board and the gas igniter is minimal. Just do not try to power a DC refer off this connection. Not enough AMPS and you will kill it.