If the tach is sealed, you will somehow have to open it up,(backplate removed like Angelis suggested) in order to rid yourself of the moisture, other wise heating it up will evaporate it, then it will condense again when it cools down. If you can open the tach up, then heat then try to fill the tach with nitrogen you might completely fix your problem. I have removed moisture from headlight bezels on cars this way, then filled the bezel with nitrogen and the moisture doesn't usually return. If you don't have access to nitrogen locally, purchase a whip cream maker that uses nitrogen cartriges as a propellant, pressurize the empty dispenser and fill your item with nitrogen.
