Take the panel off under the dash on the side that is leaking, and the carpet out of the front floor area. Then lay under the dash with a flashlight while a friend sprays the car down with water. Pay close attention to the windshield gasket area and for rust holes in the firewall.
Hope this helps.

------------------
MB Medic
14 year MB tech
7 year shop foreman @ independent MB/Volvo shop