I have the Dewgard DG-1, unless you make modifications to the circuit board, you can't control it externally.
If you have the DG series of heaters, you can preset the sensor to operate more often, I turned mine up and have left it at that setting year round without any dewing problems.
If you have your mirror in a plastic holder, I would suggest making an inspection hole.
I did this on my 12" Lightbridge.
