Not sure about the Dodge? but my Duramx block heater wont even turn on until -20 even if its plugged in, (which it hasn't hi yet).
I don't plug mine in until it gets that cold out.
thats strange, i thought the duramaxes had regular plugs so they work at warmer temps... if not just cut the plug off and wire in a new one, did that for a friend and dummy started plugging in her Tahoe at +0 temps and after she was having issues...