300E water leaks

My '89 300E leaks into the driver's floor. The AC condenser drains are not the problem. Running the AC for long periods leaves no water, and both drains under the car drip when I park it. I don't believe yet that it is a windshield leak, since it sat in a hard rain for several hours recently with no leaks. It seems to come from driving in the rain and comes from above, I believe, having put towels in the floor and up the firewall as high as the pedals. The water seems clean, but does seem so far to come from above floor level. My shop has cleared drains in the sunroof and one under the brake booster. Where are the drains in the sunroof located and what is the best way to clean them out? Do you have any other ideas on finding this leak?