Solution from GPSOz
Mate, I called the retailer (GPSOz) & told them about the problem. The guy I was talking to must have looked it up on his computer via the Garmin Knowledge Base. It was a known problem with the Montana, and he suggested that I connect up the power cable (which I still haven't done) or put some tape on the battery.
The seal didn't seem affected, and there has been no dust ingress into the battery compartment. Can't offer any advice on the earplug solution...
Tape solution solved the shorting out problem.
So far, so good...