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the aux rad fan helps to force cool air through the fins. it is a good thing to have.

a rad flush won't check for leaks--a pressure check will. i'm not familiar with these places--you may want a mechanic to play around under there.

check the wires to the fan and check for corrosion at the connectors. also check your fuses. the thermostat (or sensor) could be an issue also--in that it isn't sending a signal telling the fan to kick on.

a side note, check the passenger side floor board for any fluid leak, if your shutting off the truck, and you smell the fluid inside--you could have a heater core leak --possible.