For that you will have to take down the back trim. There are How-to's on the subject. But, be practical about it: if the belt is dirty in the area that NEVER gets pulled out, it doesn't make any difference, does it? You will NEVER see it and you will NEVER use it (that part, I mean). Your problem is friction of the belt against the guide at B pillar. After you do as above, it will go in smoothly.
I would suggest to try this first, and if it checks out ok, turn your attention to other parts of car.
If you insist on taking down the back trimcards, you can find some procedure in my thread, although I am not sure if images will show since photobucket acted up some time ago.
You do have the complete TIS online and there is the complete procedure.
https://workshop-manuals.com/ford/puma_1998_06.1997-12.2001/mechanical_repairs/1_general_information/100_service_information/100-00_general_information/description_and_operation/about_this_manual/