Rear nearside air vent

Whilst washing the GTA over the weekend I noticed that any water washed down the nearside air vent appears out of a slot in the bottom of the door sill.

Presumably the area inside the sills is untreated? (So probability of rust issues here is high)

Was thinking of removing vent and pouring down a load of Waxoil to help protect this area.

Offside I presume diverts all air and, come to that, water down to the air box.

i think if you remove the trim inside the car, you will see that it drain down inside the bodywork and than through the sill drain.
In principle it may seem bad, but body panel is plastic and painted.

No.

Unless you want to pour 30 liters in each sill

The metal is just a 5" tall,2 mil plate with a 1" return under the car .All you be doing is oiling the fibreglass unless you fill the whole cavity between outer fibreglass and plate.

Here's one just come in that has had some previous repairs This is looking into the sill cavity through what used to be the front outrigger where it attaches to the metal sill .

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What is worth doing is drilling an extra drain hole in the sill so that the water that gets in via the door push doesn't form a lake under that area until it locally fills the cavity and then flows over and under the rear seats ........

So are you saying that the outside part of the sill ( ie the part you see when you look at the side of the car) is fibre glass?
I always thought the whole sill was metal and was bonded to the body.

Not sure, but i think there is pictures of metal structure in the work shop manual.

Yes.
Cut through outer fglass to do repair as here .

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Makes me feel ill looking at that photo and thinking what mine is really like behind the glossy finish.

Do the internal metal part of the door pillars join the sills?

I had in the past Waxoil'd the door pillar by taking out the door switch.