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So the Heat and Air work fine on all speeds but when the temp gets really low out side , all the sudden the blower motor will not turn on sucks in the morning waking up frost on window and defrost will not work or blow for that matter anybody have a clue why it works on a normal day but not a really cold day?(f250 5.4 liter yr2000)
I would pull the blower motor resistor and see what kind of condition it is in. Not sure about the 5.4, but I know on my 2000 7.3 it was a 2 minute job and was in bad need of replacement.
While your there, check the contacts on the plug and see if they are in good shape. Maybe the cold is causing them to have an intermittent connection. That can be replaced if needed too. Someone on the 7.3 forum just did their plug and resistor about a month ago.