Having just completed this repair, thanks to Lippo, I thought I should add something to the topic.
The cause of the problem appears to be,
1 The use of the switch acts like a bellows and sucks air in.
2 As the metal plate cools it allows the water in the air to condense out.
There's not a lot that can be done about 1 but you could replace the metal flap with a plastic one (as I have done). I will check in a few months to see if there is any water there and report back.