I think if the instrument bodies were metal it would help to transfer heat from the underdash into the instruments, and might keep them drier. Certainly I never had this on my MG Midgets.
On my Roadster I've added clear plastic film to the small gauges, but that alone made little difference.
The thing is, overnight they clear. Park up and they clear. It's all strange.
Just theorising out loud
Do you think it’s to do with the fact that the cockpit heater is set so far back under the dash so when we are driving with the heater on, it’s warming any moist air which rises up under the dash into the instruments?
The instrument glass being further forward and higher remains cool as its exposed to the cold upper cockpit draft so the heat differential is greater
