the B post...

the bottom fixing base of the rear wheel arch....

and the buckle anchors, under the base of the back seat...

some cars don't come any rear belts, some with only a single lap belt and others with 3 point belts. There should be plastic bungs in the anchor points if they aren't / haven't been used.
I don't know if this also applies to series 1 and series 2 cars, perhaps someone can confirm?