To add to SB post..
If one jumpers the high side pressure sw at the reciever/drier and the low fan works , than the integrity of the a/c fan circuit has passed the test.. so , the common suspect is now low refrigerant..[ or , in rare case , bad presure sw]
This is an easy test when one does not have gauges to verify charge/high side pressure..
Do not expect an a/c aux fan on a cool day , as ambient temps directly affect high side pressures .. [ie.. high side may never get to pressure cut-in spec of switch when amient temps are low.. cool day defrost position is an example of this condition...a/c working without aux fan being called for]