When I owned a 1984 110 Station Wagon, there where nylon straps on the side of the seatbase that held the lower end of the seat belt webbing. These were held onto the seat base with a screw at each end through an expanding tab. One of mine broke/snapped, and when searching for a replacement found that they were not listed in the parts book !
A similar arrangement could be made using a strip of flexible plastic (wide cable tie).
My vehicle had the type of locks with the plastic shrouds fitted to the body pillars.
We used this same type of lock to upgrade my brothers S111 109 SW with great success - no more slamming of doors.
