Had the same problem last year on my Bullet. They had actually built a nest inside of the furnace and it caught fire! Luckily I caught it before it caused any damage. I pulled the furnace out and sealed all of the openings with aluminum grill from a screen door, bought at Lowes. I cut it to fit and pop riveted it in. I absolutely hate them, this year we didn't have any sign whatsoever. One other thing to keep in mind, I think they were coming in on my shore power cord. Last time I winterized it I sealed that opening with steel wool. Good Luck