Had this happen to both front belts recently. This fixed them and they work better than ever. Sure the rear is similar.
Start by removing the bottom door sill and then if you put the seat down the large side panel should be able to be pulled off.
Now you have the issue of repair or replace. More than likely the clock spring has either broken or just come loose from it's outer keeper. It's encased in the white plastic piece. Easy to remove and take apart, just don't let it pop apart!
The inner piece should spin freely making spring tension since the outer end of the clock spring is affixed by just being bent around the keeper tab. When I took mine apart the spring was rusted up real bad, Broke up the rust with PB Blaster and packed it with grease.
If I remember correctly, there should be a total of 12-13 rotations before the spring runs out. I found that the spring needs to be pre-loaded with 7 rotations, leaving 5-6 rotations for the belt to pull out. There's a small pin hole in the center of the plastic case that will require a small allen head or nail inserted to hold the pre-loaded spring while you reassemble.
Hope this helped. Wish I had pics, this was all just from memory. Good luck