My suspicion is the openness of the dash even if fitted with an undertray, you jump in for a winter drive and all the gauges tubes, wires, vent holes are cold, your legs give off invisible warm water vapour that heads upwards and fills the back of the dash area looking for somewhere to condense.
I wonder if a tiny 12v 'puter fan hidden behind (that's energised by ign on) would help stir up the damp fug ?
I think that a fan has potential, but the ducting would be a faf to get right and then it depends where you source the air from. My Roadster gauges have a wide leather apron over the top, and carpet closure under the dash, and they still mist up. So it seems that we need a cold air feed through the gauges........
