I think if your auto's dash was exposed directly to the outside air, you'd have more of a problem. The air temperature behind your dash is very close to the temperature in the rest of the car, so condensation is not likely. If there is warm moist air inside the dash, it probably escapes out the top of the instrument panel since heat rises. On the other hand, our tractor's instrument panel lays almost flat and the topmost part is the clear plastic cover. When we turn on our tractors, there are "idiot" lights which come on and warm the air slightly. Also, if a tractor is kept in a garage, it's likely to be warmer inside the garage than outside. When we go outside, suddenly the air is much cooler than inside the garage. The warmer air inside the cluster rises up to the clear plastic cover which is now cooled by the outside air. Condensation will occur at that point and the amount will be determined by how humid the air is inside the instrument cluster.
The only ways I know to prevent the problem are:
1) Warm up the clear plastic and keep it warm (aircraft use heated windshields to accomplish this). 2) Let the warm air escape from the instrument cluster. 3) Pump a vacuum on the cluster and then fill it with dry nitrogen followed by a hermetic sealing. 4) Don't put your tractor in a heated garage. Leave it outside. You'll still get condensation, but not as much.
I leave my tractor outside or under a carport, so it stays at the outside temperature. I have only minor condensation inside my instrument cluster.
That's my nickel's worth.../w3tcompact/icons/smile.gif