Funny, Harley Davidson gauges, being that they were exposed to the elements of all types fogged up and many questions were lodged across my counter about it. Harley's reply stated that there were holes that allowed air to enter and defog over a given time.
Actually on the cars they had a small vacuum line from the intake manifold to the bottom on the tach housing that pull the moisture from them. It was small like the line that went to the heater box.