Jumper the pressure switch on the reciever/drier [ the one with the pig-tail leads ] with a/c on and see if Aux Fan comes on.
If Yes , fan cicruit is OK , you have a low refrigerant or pressure sw problem
If No, Take six screws around fuse box off to get to relay compartment behind fuse box There are two fan relays , Look on top of them ..possible fuse problem....
If No, check connections at dropping resistor behind driver headlamp.. this if part of aux fan feed circuit to fan motor/s..