Those instruments are sealed to only keep dust out. To keep air out there would need to be a hermetic seal. That might happen on a space shuttle but probably never on a tractor. Now we're back to a couple of solutions as in reply #2. Changing the moisture content of the air to lower dew point temperature would be impossible in this application. That leaves us with the second method of raising the surface temp. of the lens. A question: if the gauge cluster is only lighted when driving lights are on we might consider leaving the lights on to keep the gauges lens warmer with the heat given off from the bulbs.