Seat belt mechanism on BMW 520i

Hello everybody,
I have a little problem with my seat belts on my BMW 1990 520i. I was chopping down some trees in my garden and I loaded up my car ready for the
tip. I noticed afterwards however that one of the rear seat belts would click into the socket. I presume that a leaf or a bit of pine cone from the trees must be jamming it. Does anyone know how to access the mechanism?
Many thanks for your time,
James Sherlock

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