You know the little packs of Silca you get in a new pair of shoes?? They say "DO NOT EAT". Put a couple of them in the safe, they will absorb extra moisture and then put a small 50 watt light bulb in the safe, and leave it on. That will take care of the problem. I wouldn't use a heater, seams a tad unsafe.
I never saw a wild thing sorry for itself. A small bird will drop frozen dead from a bough. Without ever having felt sorry for itself. -- D.H. Lawrence