Was there ever a resolution to this? My '85 300D just had the same problem this morning for the first time that I'm aware of. It's been raining here all week and no issue, but this morning I had a pretty nice stream of water snaking its way out from between the A-pillar trim and the horizontal piece it contacts at the top. It stopped and started once or twice on my commute, but I wasn't able to notice any consistencies to help diagnose.

I know my sunroof felt/gasket is old and tired, wondering if it suddenly let go in the last spot it had been holding? I'll try to snake the drain holes as mentioned. I'm not sure it would be the windshield for me, as the leak (I think) was above/behind the windshield.