If you hookup and drive away, not spray and kill them, they will not be dead inside the furnace. They will all fly out. When you get back, park in a different location for awhile. That's what I did. Not sure how long they were in mine or what type of nest they built. I fired up my furnace and everything was fine after a little smoking. That was 3-4 years ago.
Also got them in the wall of my house and attic. I made a trap out of pipe and plastic drink bottles. Took a few weeks but I caught them a few hundred at a time. Then I sealed up the opening they were using
2014 Silverado 1500 5.3L maximum trailering package (yes, I'm towing the 34')