Leaks on our old SAAB 900 classic convertibles is a very common situation as the cars are now at least 25 years old.
It's a complicated subject but I will try and pass on my experience.
The 1st mistake many have made is that they thought the main body (the area you porimarily see is leaking) This rarely allows moisture past it unless it is vandalised/damaged.
This area allows water to pass INTO the rear wings and it is supposed to. Thus if the drain holes are blocked then water will fill the rear wings and then pour over into the rear and front footwells. There is no option really but to remove the interior and thoroughly dry out the area in addition to ensuring the drain holes are unblocked.
Another area where water can find it's way in is via the pollen filter housing - it has a drain pipe on the bottom that is a sod to put back on but it must be cleared of debris or water will fill the pollen filter and then come in via the floor heating vents. It can also come in via the mounting bolts for the knee roll that are visible with the bonnet open and looking in the area where the wires go through the door pillar.
I'll post pictures later.
I will also in my next installment discuss the issues relating to the rear window and the water ingress there. This is the most common complaint owners of SAAB 900 classic convertibles have reference water ingress.