Sorry, didn't mention it's a 1997 Corolla.

Dennis
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probablly directly infront of the passenger side right below the window...

I had the same problem and solved it on two Corollas. Don't Taze Me,
Bro! is correct.

Remove the plastic trim between the windshield and the firewall on the
passenger side. You will see a couple of intake holes on the passenger
side in the firewall.

That is where small animals (mice) can get into the HVAC system and
create nests and defecate on the top of the cabin air filter and ducts.
Not a pretty sight or smell.

I solved it by cutting pieces of stainless steel mesh (3/8" square hole)
a bit larger and the same shape of the air openings (there were two),
and using silicone sealer to adhere them to the edges of the openings.

Problem solved 100%.

It's a tight area to work in so take your time.

The only other solution without using the mesh is to push the
recirculate air button on the control panel so as to put in place the
plastic air dam to close off the outside air openings (it will block the
openings to the fan motor). But I could never remember to do it all the
time.

I would hope that Toyota would have recognized this issue as a problem
for owners that live in rural areas and install a factory fix, but
nothing yet.