Rear Seatbelt Jammed

Hi folks

having a little issue with my rear seatbelt. It’s fully retracted and jammed. There’s also a bit of plastic on the belt that I think should be in the retractor assembly (pictured).

i’m guessing I’ll have to take the assembly apart to fix it, but I can’t get to it (can’t figure out how to release the correct bit of plastic covering it).

So far I’ve tried:

Folding down rear seat to see if I could get extra slack that way

Rocked and shaken the car to see if I could vibrate it loose

WD40 sprayed into assembly

Swearing at it.

Anyone cot any hints, or could explain how to get at it please?

TIA

Mark