I'd be caucious to leave anything exposed outside of the pass compartment, two tricks: spray red battery terminal grease on all outside connectors and seal all crimp terminals with RTV or liquid electrical tape (or dip-grip).
Some electrical components are best obtained from the marine supply store since they're intended to be exposed to water.
One question was the rubber cap on that fuse holder checked recently? Once it's replaced check for heating of any of the wiring to the fans under load. Doesn't sound like anything outside of the fuse holder caused the meltdown. If you can find one get the larger style blade fuse and holder (about an inch long) the blades on the standard fuse are too wimpy on a 30A circuit. If you can dig that fuse out you'll see one of the connectors overheated.
"if it heats up at all, it's too small" this is the golden rule of wiring.
Your fans will run faster as well when you eliminate the 2v+ voltage drop the wiring may be causing.