The most likely source of your leak is behind the Battery. I just got finished cutting , shaping and fitting my second firewall patch because the Battery acid had eaten a hole threw the firewall. This is a common problem on the 123 body styles. The hole in my 240D was only about 2 inches square but this latest one in my wifes friends 240D was more like 4 by 10 inches. Plan on the better part of a day it you are going to try this repair yourself.
Good luck