My old "gun safe" has 2 golden rods (& a viewing light), the wire lead in through the threaded hole in the top. My smaller "safe" safe does not have an access hole to wire it. So far, I've not had a problem but I'd really like to get a Golden Rod in there. All my guns are wiped down with Break Free. I tried & failed to get a small flat cord through the door. The basement has a dehumifier in summer & a wood stove in winter... but is not as dry as I'd like.
I think the Golden Rod type is the only way to go... or a light bulb (poor man's heater).
Russ