Your best bet would be to use 2 thermisters, one for the ambient temp (air on) ad one on the fins (at the air on location). Remember that your liquid line is going to be X deg colder than ambient so you will need to start monitoring when the ambient is X above 0C (maybe 3-4C). Turning on defrost is not the hard part but knowing when to turn it off is, and that is where comparing the two temps helps. You can set the differential to determine when you think there is no more frost.
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