Have you checked the rest of the interior of the car in case there's moisture elsewhere?
Take the F-Type out for a spin with the rear heated screen etc on.......that’ll take no time to clear the condensation.
You never know, perhaps when you’ve sealed the tailgate rubber it may help a little?
You could try swapping the parking spot with your second car, or try parking the F-Type the other way around to see what happens then? Light, wind, rain etc could cause some different effects?