How watertight is your roof and A post gutter ?

[GTS290N]

There are a bunch of drain holes across the bottom of the plenum, so water shouldn't pool there.

[GrahamFountain]

I think it depends on how quick the plenum fills up - it's in Lancashire where it rains a bit.
It may also be affected by the battery terminal cover, at least one "spare" thermostat gasket, and a collection of self-tappers, washers and other sundries (not sun-dried) I can see, but can't get out from under the wiper motor.

Graham

[GrahamFountain]

The final leak was from the windscreen rubber running down onto the corrugated tubes from the plenum and then onto the shelf. I've now fixed that with this (https://www.ebay.co.uk/itm/SILICONE-LIQ ... 2749.l2649) thin as water clear silicone sealant that seemed to seal it on the first application - though I've given it three in total.

Don't know how long it will last, but it is a big bottle and less than half has done 3 treatments on the Doly, 2 on the Herald, and one on the wife's PT, which didn't leak but's been making creaking noises since a new screen.

Only problem with it was you cut the only lid they supply with the bottle as a spout. Fortunately I found a lid off a bottle of vape that fits.

Graham