The function of the auxillary fan is to come on when the a/c pressure rises to a certain degree. The other function of the fan is by a temp switch located on top of the engine , when operating temperature of above 110 C the fan will come on until engine cooled down below that stage. So the question is , does your fan not come on when operating a/c sitting in traffic , which you will feel the a/c not cooling efficiantly also.
The fan will not stay on all the time the car is running ( that would burn up the resistor and possibly fuse ) It cycles with the a/c. Now if you know where the reciever drier is you can turn the key on , jump the two wires at the high pressure switch ( red or green switch) and fan will come on. Let us know what you find.