The dehumidifier will be very ineffective in a cold garage because the way it works is identical to an air conditioner except the condenser is downwind of the evaporator. In a very cold garage, the evaporator (cold side of unit) will not be able to condense enough moisture out of the cold air to make much difference and the unit's metering valves will limit the unit's work output (to prevent the evaporator from freezing over) so the condenser side won't get much warmer.

A simple heater will be far more effective. As somebody above correctly pointed out, simply raising the temperature in the garage will dry the air out since the relative humidity will drop as air temperature increases.