1990 gmc 1500 5.7 l 4x4 heater system fan go/no go randomly

friday it fails to start on ignitition, saturday afternoon its back (yeah!!) since it was -20c. now its no go again? checked coolant levels all fine. and doesn't come on randomly if I try restarting over
and over, anyone have ideas to the key of the failure or the trigger to get it back on temp?
Thanks, Rob
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