Here's another pic taken from under the car looking up
through the hole in the LH lower plenum chamber (i.e. the tube exit hole)(which is out of focus). It shows there's rust around the hole in the upper chamber as well (i.e. the hole in the floor of the ECU/pollen filter chamber) .
This proves that water is running down the
outside of the rubber drain tubes and causing rust around both the upper and the lower holes.
I whipped the cover off the LH plenum and here's the view looking down through the ECU/pollen chamber to the lower chamber beneath.
Here's the tube with the water trail (& muck) clearly visible on the outside.
It doesn't help that it's turned into an oval (!) when the hole it's supposed to fit is round.
So this is what's happening on the LH side: water runs off the windscreen and drains down under/through the foam strip that's supposed to seal the little plastic bulkhead thingy. Then it runs along the upper chamber floor behind the pollen filter and forward towards the hole. The oval rubber tube doesn't seal the round hole properly so water travels through to the lower chamber on the outside of the tube - it can't drain out through the exit hole so it just sits there.
Exactly the same thing happens on the RH side, except there's no plastic bulkhead because the wiper mechanism is in the way. I can't do anything about this side without taking the servo out - or taking the closing panel off.
The sealing /waterproofing of these plenums really is lousy!