On my old Zaf, the heater O rings inside the cabin were perished. Water was leaking out, down the back of the carpets and out under the floor. I only noticed when I parked on an incline and the water started dripping out of the floor pan. Was a right bugger to do as well, had to remove loads of trim to get at the O rings. We double ringed the joints when we got to them because we didn't have precisely the right size O rings to hand. Never leaked again after that. Although shortly after, the water pumped housing developed a hairline crack and water leaked out under pressure. Radweld cured that :-)