Originally Posted by meltedpanda

others may chime in but it is my understanding that "defrost" is the fail safe built into the ACC pushbutton unit. If you cannot jump start it then I would lean to the pushbutton unit, and yes if you have power to the fan(past the regulator) then it is probably OK.
One other thing to check is the temp interlock circuit. should be a one prong temperature sensor somewhere around the thermostat housing. At least that is where it is on my 300SD. If this unit fails it will not tell the ACC to come on. It senses when the coolant is warm enough to allow unit to come on.
Try unplugging, turn ACC to all cold and see if comes on.