Have you considered perspex /polycarbonate rear window..you could then drill holes in window covered with rain shields as per many race cars.
My experience of the S5 hardtop was that once we'd got it watertight, to prevent misting I then needed 4 or 5 vent holes in the trailing edge of the roof. What Kevfitz said :-)
Interesting, do you have any pictures of these? I didn't realise they were needed on the S5.
I can grab pics when I'm next with the car... I needed the through flow of air that the rear holes provided only after the roof had been fettled to a point where *it doesn't leak*! (Well, the header rail doesn't leak, and the vertical door seals don't leak... the bottom edges though aren't yet watertight, but it takes a deluge to cause seepage.) At that point of watertightness, it's also sufficiently airtight that unless a window is cracked open, then two-up on a damp day the side and rear windows mist up, even at speed.
Williams drilled 4 or 5 holes in the roof's trailing edge - which the S1 doesn't have. On the other hand, does the S1 have heated windows?
Will
Interesting that it would do that, have you tried running the AC as that is a great way to remove moisture even if its cold and set to heating. My S1 has heated glass all round, so it will just be the rear screen that isn't (unless I can source a heated rear GTN screen).
