We had one a few years back, it was coming in through the attic fan. We put a metal screen so it couldn't get into the attic. Maybe 3 years later it came back and started nesting betwene the screen and the attic fan. One week later we replaced the attic fan with one designed to keep squirrels out. Good thing it was the winter or we'd have squirrel pieces everywhere.
The squirrels out here always climb on the power lines, thats how they got into our attic. Interestingly enough, once we shut them out, I think they moved next door. Squirrel traps work good, just try to get it up there when they aren't around or they can get into your house....