Water leaks, '89 300E

In my '89 300E water leaks into the driver's floor. I have covered the windshield seals with caulking, hoping that the windshield, the usual suspect, is the source of the leak. No luck. Perhaps I didn't seal things right but I tried to cover every inch of the seam where the rubber meets the glass.
It leaks only when driven through a heavy rain: it will not leak if left in a down pour for hours. If further tests of the windshield still don't find a leak, can anyone suggest where on the body there might be a leak? From what I can see, all the body drains are clear and no one can find any place in the firewall where there might be a leak.