That shit'll buff right out.
I have a sinking feeling that the slight 'rattle' coming from the vent fan is actually a dead critter caught in there - wouldn't be a huge surprise, given the nest material we found in the engine and cabin air filters. I've seen plenty of pictures of that kind of carnage on the 'justrolledintotheshop' subreddit, but until I get a Chilton/Haynes manual (I don't anticipate doing enough work to warrant the $$/$$$ for a FSM), I won't know if it's a 'fun' weekend project with the kid, or a drop-off at the mechanic.
But the good news is that the new head unit did drastically improve the sound through the OEM speakers...so I've got that going for me, which is nice.
Your license plates. GIVE THEM TO ME
plates | pints | plaid on YT: https://www.youtube.com/@platespintsplaid
You'd really need to look at the shop manual and get a sense of where the cabin fan is. If it's like, say, a MkIV VW it's a 20 minute operation to pull the fan. If it's more like a Saab 9000, plan on two weekends. Since none of this work is ever actually difficult or messy and you don't need the car this instant, I'd really consider taking on the project. It's a good lesson, good bonding time, and you can work on it at your leisure. Maybe also a good reason to order the real factory manual (ebay, even) and get familiar with reading it.
That said, where was the nest in the cabin filter? Before it or after it? If before it, and the filter itself wasn't perforated, how would an animal get in there?