Just in the ac vent. I plugged hole around wipers and two holes in the far corners. Made nest on squirrel cage fan blade. I think if u choose only interior air the door in vent shuts to keep them from getting to fan. They can can get in car if they are in fan and you change to only interior air. They can also get into air intake under air filter. Nice little house for them. If the fan sounds funny, it may be mouse nest. If you hear thud, the fan is batting mice. They like to rip off insulation on firewall. Don't go camping without closing access to ac vent.