On Monday, December 4, 2017 at 8:14:41 AM UTC-5, harry newton wrote:
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?

Others have given some tips on how to figure out if it's the speaker.
Another idea is to hook up some other speaker, eg one from a stereo
system and see what it does.

For speakers and what will fit which car, Crutchfield has always been
a good source. If you figure out what you want, Ebay is another source.
But if buying some name brand, be careful on Ebay, especially name brands
shipping from China, many are counterfeit.