I don't know if the problem is fixed yet, but I would hook an external supply to the fans with a fuse in line and start them up several times to see if they blow the fuse. The starts need be hard starts, full supply voltage, another words don't bring up the supply if its a variable. Apply full voltage so that the current draw will be at its maximum.
If this doesn't blow the fuse let the fans run for a while and see if the fuse blows, if that works I would suspect the problem being someplace else as in the electrical feed.
You may want to disconnect the fans and trigger the ckt thru the sensor that controls the Fan on, see if that blows the fuse, if it does check the wiring.
I would suspect that only the high current side of the relay is fused, not the coil side but check the wiring diagram.
This may not be as difficult to diagnose as you think. I always use an ammeter on my external power sources, but remember that an ammeter may not indicate a start up surge. That's one of the reasons that I suggested that you use a fuse.
Fred