There have been many postings regarding various interior leaks on the 80 series. I purchased my vehicle in July, it began leaking after the first major storm in October. Water was accumulating on the floor against the door sill. There was a significant amount of moisture in the vehicle, it extended through to the rear seat, the mildew odour was terrible.
My initial inspections did not reveal any obvious problems with the windshield, drain tubes or A.C. (ed - To check drain tube, pour water into that drain and check if it's draining from the cavity (right where the mudguard is). It might be clogged with mud or etc. Also check the rubber gasket inside the sunroof housing, make sure that the rubber seal is tight). What I found was surprising and could result in problems for other owners. My leak was caused my the metal clips that are used to secure the plastic wind deflector for the sun roof. The clips were binding on the rubber seal for the hatch. The binding resulted in a depression on the seal that caused water to leak in during a storm. The water flowed across the headliner, down the pillar, and accumulated on the floor.
The fix was a replacement of the hatch. Apparently the seal cannot be purchased separately. I would recommend a careful inspection of the wind deflector mounting system if your vehicle has this accessory.
(see also http://reality.sgi.com/80_series/tech/water_sills.html and http://reality.sgi.com/80_series/tech/window_leaks.html)