Assuming the fan runs in high speed mode but not low, most likely its the resistor.
If you have a Durametric cable, you can activate the fans in low and high speed. If the one that is NOT working in low speed, runs at high speed, then its the resistor of the relay.
If you don't have a Durametric, then you can try a few other things.
You can test the relay by switching the low speed ones one for the other. If the fans operate at before with the relays switched, then it's not the relay.
You can test the ballast resistor. Here's a link that show how.
http://986forum.com/forums/general-discussions/38026-c-fan-speeds.html.
If the fan doesn't run at any speed and you've switched the relays and the working fan still works, then the motor is the more likely culprit.
Good luck