TBJ has a good point.... It wouldn't be that hard to get an inline thermostat that closes at that temp, i have used a few for fan controls and such that close at a certain temp range. Anything below freezing and the thermostat turns them on when the ignition is on. You could put the thermostat inline in each door and the sense side of the thermostat against the cold outer skin of the door would do the job with a bit of thermal epoxy... Then i wouldnt need wires to the defrost switch in the dash just a fused power to each door...
Anyway just thinking out loud here.....
now that is WAY WAY beyond my skill set.