There's a chipmunk family playing Die Hard in your ductwork. How will you find them?
You've seen Alien? Then you know how hard it can be to catch a squirrel that's moved into your drainpipes. You've seen all the Alien sequels? Then you know how frustrating it can be when the squirrel keeps getting away to try again later. Because it can hear you coming. And you've got no idea where it is.
So the smart thing to do is NOT to get a flamethrower and crawl through the man-sized tunnels. The smart thing to do is to get a camera and some extension cables and do a little recon. Maybe figure out where the squirrel lives. Figure out what the squirrel wants, how it thinks. Hey, you know the speech. You've seen Caddyshack, right? Same principle.
With the ability to see into the squirrel's domain, you'll have the power to strike at the squirrel when it least expects you. That way lies victory. The alternative is just bumbling around smashing your pipes and drains. Who suffers then. HINT: not the squirrel.