Chevrolet Repair: 98 k3500, heater fan, blower motor

Question
What would make the heater fan quit working when truck is shifted to 4wd?

Answer
GOOD MORNG DENNY,SOMEKIND OF A SHORT..THIS IS NOT COMMON AT ALL. THE BLOWER MOTOR CIRCUIT IS NOT RELATED TO THE FOUR WHEEL DRIVE CIRCUIT AT ALL...DOES THIS HAPPEN ON ALL SPEEDS OF THE BLOWER MOTOR ??? ARE YOU SURE IS IT NOT SOMETING LIKE A RESISTOR NO POWER AT LOWER SPEEDS ???
I WOULD CK THE BACK OF THE FAN SWITCH MAKES SURE ALL CONNECTIONS GOOD AND TIGHT AND LOOK DOWN AT THE BLOWER MOTOR ITSELF AS WELL....THIS IS ALL I CAN THINK OF. LET ME KNOW IF YOU FIND OUT ANYTHING ON THIS.  GOOD LUCK TO YOU,TOM..