This is a manifestation of the instrument temperature being below the dewpoint of the air behind the glass. Inside my garage the humidity is currently around 80% and the ambient temperature is 5C. At those conditions the dewpoint is just under 3C, but the instruments are at ambient temperature so there's no misting.
Get in the car and start the engine to warm it up, the interior will heat up quicker than the instruments (they will lag behind), so let's say the car gets to 10C, relative humidity is unchanged, the dewpoint is now just under 8C, but the instruments are still around 5C. They have no choice but to mist up.
Will a low wattage heater in the car help? I have one in mine, but not for that purpose. I suspect that unless the temperature of the inside is kept in the low teens then misting would still occur. Or drive without any heating in the car, that should work (if you can stand it).
I'm not in anyway questioning your observations and explanations Of all the many many soft top cars I've had and some have been outside all year round, many garaged (but used all year round) creating all the dewpoint conditions you mention I've never had from my humble MG Midgets to my Mercedes convertibles (currently 9 and counting) and my Porsche convertible, ever had their instruments fog up. Yet every Morgan I've owned has this characteristic, and the MG Midgets had the same brand/maker of the instruments (from memory). I've certainly suffered from internal condensation on windows especially when mohair (even multi layer insulation) hoods become saturated but the instruments have always remained dry. Just thought it was worthy of note.
