I feel for you. I had to remove the rear seat in a junk yard car to remove the rear deck to access the trunk lid rear mounting bolts. I don’t know how I did it which is why I could not answer the occasional how to question and I would not like to do it again. When re-mounting the trunk lid I cheaped out and removed and pried back some trim resulting in a few trim cracks.

You are saying that the seat belt centrifugal lever is stuck, belt fully retracted. Wife’s front pyrotechnic seat belt was stuck and I did not want to make this into a never ending plumbing job. I could only envision dried lube or little dirt and lint causing the centrifugal lever to hang up. I sprayed a little lubricant in two drilled holes and it’s been working ever since.

I have not taken apart a Volvo locking belt but in one I did take apart, there is a weight at the bottom of a rocking hook lever that locks the belt. I would try the spray first and if that doesn’t work, take it apart to find out what makes it tick. You may find accumulated dirt, a broken spring or something. At worse, you will have to get a new belt at a junk yard.

Tom