The aux fan(s) are connected to two devices.
1. Low speed is controlled by the A\C high pressure switch. This cools down the condenser and should be checked by connecting gauges to your A/C high and low side. If it is 85 -90 deg outside your low speed aux fan should be running all the time. May have bad wire going to high pressure switch or bad pressure switch or bad connections to fan. Or you can simply disconnect the switch and jumper the connectors together and move the wires around to see if you have a bad wire / connection. The Hi pressure switch has 2 wires coming out of the switch vrs the low pressure switch has only 2 connectors and no wires coming out of the switch.

2. High speed for high engine temp.

There are 2 relays behind your fuse box try swaping them to check the relay. The High speed aux fan relay does not operate as much as the low.

You could also be low on freon or have a bad compressor thus your high pressure does not get high enough to active the switch or its too cold outside.

Good luck
Anthony Bridges