I haven't worked on speakers in decades.
http://wetakepic.com/images/2017/12/04/speakers1.jpg

How do we debug a scratchy sound?
We can isolate it with the balance to a single speaker.

But is it the speaker?
Or something else?

If it's the speaker, where is a good place (other than the dealer) to get
speakers to fit a car rear deck? Are they all standard sizes nowadays? Or
is each unique?

I'm helping the neighbor's kid refurbish a beat-up 2005 Camry where I
helped her kid put in new speaker covers this weekend but the scratch sound
persisted (we thought it might have been the crud or vibration from the
crumbling melted-in covers).

The scratchy sound persisted even with the newly replaced covers.
http://wetakepic.com/images/2017/12/04/speakers2.jpg

Any suggestion on how to debug the cause of the scratchiness?
If it's the speakers themselves, are these things standard sizes nowadays?