We had some in ours a year and a half ago... they weren't IN the ducts, they just ATE the insulation around the duct... which we noticed when Texas summer hit and the (relatively new) A/C didn't work. Stupid rodents.
We put out poison, but only up high... and had to keep eyes open when taking the dogs out so they wouldn't find a 'treat'... pain in the tail, but it got the job done.
Along with closing up every nook and cranny we could find that they might be crawling though to get to the attic where the duct was... Landlord had A/C guys re-insulate and all's well... of course NOW we've got the gaping hole in the garage ceiling from The Contractor ripping the hole out hoping to expand the ceiling so new garage doors could be placed (evidently they don't make the size we have any more) and finding a support beam so went with plan B which was just closing up the garage... but didn't bother closing the gaping hole into the attic and sure enough there's scurrying... so may be coming back to this topic again soon ourselves... stupid contractor, I SWEAR that guy leaves more work behind than he fixes... job security.... not just based on our encounters with him mind, though there have been a few... but also on other's accounts of his work... like painting a house... he claims he (and a licensed painter he called) had never even heard of scraping the old flaking paint off before slapping new on... REALLY? Hmmm tangent much?