Area Behind Cabin Air Filter - What is this material?

Mine too.

I’d clean it all out (maybe blast interior with leaf blower too, hopefully stampede any “residents”), remove wipers and cowl, install 1/4” spacing galvanized steel mesh (rodent mesh) over the cabin air intake, secured with sheet metal screws. Secure end of engine air intake snorkel similarly.

Googling:

@NutzAboutBolts spark plug replacement

The @NutzAboutBolts video I’m familiar with, and the Gregg Fesco one is Prius v specific. Quick view of Gregg’s I think the components are identical. It’s just a lot of tedious prying off and wrenching to get to the firewall opening into cabin.

 

Sure looks like the underhood noise insulation, carried bit by bit through the ventilation intake and deposited to form a house.

Might not have the most curb appeal but it looks warm anyway.