Brian's response is probably right. Check the sunroof drains first.
It could also be water getting in under the moulding at the bottom of the triangular rear quarter windows. Very common on Mk2's.
A long shot would be that your wheel wells have rusted out at the top of the wheel arch, creating a hole there. Fairly common on Mk2's. Driving in the rain will then throw water up and through that hole. It will then run down the inside of the wheel arch both towards the front and the back of the car. This will cause the rear quarters to rust through behind the rear wheels and may give wet seatbelts too, but only after driving, not if the car is parked.