Two Cabin / HVAC Incoming air intakes?
I have not looked into this myself but many members say there are 2 intakes, the one on the cowl and one from under the car. If this is correct you would have to filter both.
A good search word for this topic is "mice". From the first page of this search I will quote two people:
There's another opening into that area from the bottom/side. You may have to remove the inner fender to access it though. Many install a screen across the cabin opening. I've mounted a cabin air filter there which should keep varmits out as well as clean the air.
they get in on the passenger side fire wall air inlet not the one you see by the wipers but the one you do not see down low . this is visible when you remove the top vent cover and look down on the passenger side where you will see a large opening that the blower intake is located. using some 1/4 in rat wire cover this opening .not too hard to do. they can squeeze past the blower motor fins.then come out the vents.
It seems like a lot of time to change two filters with a chance of breaking old plastic. It will reduce the HVAC air flow and worst case cause the blower speed resistors to burn out because the air flow cools them.