I have the fender liner off and noticed a small vent with a rubber flap cover by the firewall. I assume this has something to do with the heater? Is there supposed to be pink insulation inside this or is it a rodent's nest?

 

That my friend would be the pets you've been transporting for a few years. Choice of my pets was white paper towel looking material. Just get your finger or a prodding tool up there and clean it out, i wouldn't suspect any real problems, i have yet to have any issues with my car, and my a/c box was filled with seeds and other chewed up crap.

P.S. i like the avator, looks so similar. Now that i've gone full T/A body kit, i need to get a new pic.