Warm air behind the dash inside the gauge cools when in contact with th3 cold glass. More so in winter. It's how clouds form and then mist, rain, sleet, snow etc. Basic physics. Two directions to cure it would be dry the air or warm the gauge glass. Neither is easy in an open top car. I've had it on all 3 Morgans. Trads and now the Plus Four.
I wonder if a clear film over the gauge might help, the type you can get for a mobile phone screen as a protector or paint protection film. I5 migh5 just provide enough insulation to prevent the condensation.
