I have a theory on that one.
There is an acrylic glass ring between the dial plate and the glass and its purpose is to evenly distribute light coming from a bulb attached to the instrument's bottom across the dial plate. The side effect is that it creates a sealed chamber between the dial plate and the glass. Any moisture trapped in there is will condensate when instrument is lit (warm inside), and "splashed" with cold air (cold outside). Which is more or less always, as we all must ride with headlight on.
The speedo does not exhibit the same problem, as it's dial plate features two large holes (odometers, are they? ), which allow for some air circulation between the "chambers".
Drilling the tach body bellow the ring, won't help since the that will not "unseal" the "upper chamber".
Drilling the tach body through the ring won't help since this one will be sealed by the rubber grommet holding the instrument in the panel.
The only valid options (however, untested :!: ), are:
A: drill through the glass (exposing the inner workings to rain, bugs and other things that rely on gravity to assume the state of minimum energy)
B: drill through the dial plate (which may create a bright "hot spot", as the light from the bulb would now shine directly through).