I knew this guy once, who attempted to install an Electromotive EMS in a well cared for S130. He eventually gave up on trying to get it to run and brought it to me. He still had a number of unfinished details, like mounting the ECU. The EFI harness came through the firewall in the sock location, but the ECU was still sitting on the floor pan... the low point. The car sat outside for a while. Rain water collected on the unwrapped harness via the hood vent, followed gravity as its told to do, and came to a rest in the ECU connector. The car was delivered to me, dry of course. After checking the basics, I could get no life out of the unit. I pulled the ECU connector, and this very pretty, brand new ECU, was nothing but pure corrosion inside.
Moral of the story, don't make the ECU the low point.
Very few installations will completely prevent ALL water over many years of rain, under hood washing, etc. If the ECU is mounted low (in the stock location, for example), I put a 'droop' in the harness creating a trap of sorts.
Most of my installs have been OE location or in the glovebox.