Mark,
Hi. Here's some water leak/drain info. I was suprised to hear of your water source. When reading it, I assumed it would be the infamous cracked (or missing!) heater box drain hoses. There are two rubber hoses that run from the bottom of the heater box to the firewall, one left and one right. These drain water that enters that box through the large cowl vent. You should double check those hoses as well. Regarding your wiper issue, I believe there should be a space between the cowl vent and the base of the windshield where water can drain from around the base of the wiper arm posts. There is no actual "drain" for this area, but the water runs through a gap, into the heater box, and out through the previously mentioned hoses. I hope this helps somebody.
Rodd
1966 230SL Euro
1994 E420