My guess would be the deice would involve the AC to run on the heat cycle. Part of the AC function is to remove the humidity from the air. Running the heat through the AC evaporator would hit the windshield with warm, very dry air, and would probably remove the ice faster.
Most of the new stuff automatically runs the defrost through the AC evaporator.
my 87 Diplomat had that, but if you pulled the pushbutton out you'd get defrost without AC. I can;t remember if my 72 Dart had that, but my 65 Cuda did not, till i retrofitted a later control module in and now it does.
