2002 cvpi strange smell

Hi all, I have a car that's been sitting for a while and it recently has acquired an odd smell in the interior. I've checked everywhere and febreezed the heck out of it and my guess now is something is dead inside the vents. Is this common and any suggestions on cleaning it out? Thank you

Mice love the blower motor area.

how do they get in and what's the best way to get them out? Shop vac?

pull the blower motor out and inspect for a nest - look into the plenum as far as you can and inspect. may need to pull dash to removed heater case if you cant find the critter.

kind of what I thought. I'm just still wondering how they get in. Thank you