The aux fan will come on for 2 reasons: the pressure switch for the a/c tells it to come on, or the engine temperature switch tells it to come one. I think the engine temp switch is at 105 degrees C, and the pressure switch for the a/c is located on the a/c dryer, and comes on when the a/c is turned on. Try jumping across the dryer switch and checking to see if this makes the fan come on, with the ignition on and a/c turned on.
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)