Ford Fiesta mk6 noise coming from rear.

[klaudmjj]

For months now I've had this squeaky noise coming from the rear of the car. It didn't really bother me at first as it was quiet and music would just make it not noticeable.
However now, the noise got louder and louder as time went on and now I can hear it even with music unless I max out the volume.
I found the source of the noise and it's coming from the cover where the rear seatbelt sits. Any idea on how to fix it? The other side does not make that noise.
https://www.youtube.com/watch?v=hQ-URBK6238 here is a video.