The plastic covers over the metal babyseat anchors on the rear deck rattle with the bass from the subwoofer, they have noticeable play of about 1mm each. What can I stick in there to quiet them up?

 

Blu Tack? Some small strips of double sided tape?
I've never come across these things, they were not required on UK cars, but have used the above items to good effect to stop rattles.

 

Hmmm.. I didn't know that. I really don't need anchors in my car. One day I might remove them and have a shop put in a nice piece of "carpet" in place of the factory stuff. I think that would make the rear shelf look nice and clean.

 

Adjust your audio.

 

I did, subwoofer = 7, bass = 7, lol

Stu, great suggestion! Bluetac worked perfectly, thank you - I had the 3M version sitting around in off-white color (it's somewhat visible in the seam)