I have one of those heated rods....works pretty well. I live in a humid basement and my guns dont rust when there in the safe.
"Heated air is still humid air,"
Sure your heating the air in the safe, but your also heating the guns in there too.... So it keeps the water in the air from condensing onto them like a cold soda. With no water your guns you dont get rust. In theory anyways.
Your guns dont need to be super hot, just warm enough so the humidity doesnt condense on them.
Also, I have 2 dehumidifiers running in my basement 24/7 as well. Its so bad, I have set them up with drainage tubing so I dont have to empty them. I cant keep up with it. Theres a hot shower in the basement that adds to it, and a dryer. Gets bad sometimes.