So since you said you're not a mechanic, perhaps a brief overview of what the heck a fuse actually does might help you in troubleshooting?
Basically, the fuse is there to protect the wiring. if you get a short, that short is literally a shortcut for a bunch of surplus angry little electrons to bypass the load (in this case a nav light) on their way back to the ground side of the battery. Like mike said, a bunch of extra resistance from a loose connection or corroded terminal can cause the system to try to suck more amps to overcome the increased resistance as well.
The wire is sized to handle the load designed in the circuit, so a bunch of extra amps cruising through that wire causes it to heat up and melt/burn/smoke/whatever.
The fuse in the circuit is sized so that it can handle the normal load, but will pop (burn open) at an amp rating slightly higher than the circuit pulls in normal operations, but less that what would allow the wire to heat up.
Therefore, if you're getting a popped fuse, it means that the safety feature is doing it's job. Bent pin, wire with damaged insulation, corroded terminal, whatever it is, it' something thats causing more amps to flow through there than the circuit can handle so the fuse is popping.
If it's intermittent, that just means that the short isn't happening all the time. Like maybe a damaged wire is flopping around and bumping up against a rib flange every once in a while. Heck, years ago I had an old motorcycle that would blow the headlight fuse seemingly randomly and it turned out to be a cracked connector housing that was allowing water intrusion in there when I washed the bike. Water is conductive, and if I turned on the headlight before it evaporated back out of there it popped the fuse.
Hopefully this helps as you trace this out.