From my experience >> rainwater inside the cockpit primary cause is a leaking windshield gasget... I currently have this very problem with my 80 911 SC >> but since I park it inside hte garage >> and don't drive it in hte rain >> no leaks..

my BMW 2002 had a laek >> after 20+ years >> hte rubber loses it flexibility > hardends and water will find a way through

you migh try to checkit by having someone hose down the windshield while you look inside >> may/may not work >> anyway my money is on the windshield gasget