It was a while back but I think it goes like this . The seat belt mechanism sits on the inner wheel arch .You have to cut a hole through the trim at the side of the parcel shelf (the hole size is imprinted on the underside of the trim) .The top mounting point is on the inner c post above the hole you have just cut . if I remember right. Its got a plastic plug in it .
On my 88 Sprint (33 mechanicals) the top mounting points for the rear seat belts are located on two small rear shelves just behind the rear seat and next to the C-pillars. I guess the suds are slightly different and located direct to the c-pillars.