Disconnect the fan and see if the fuse blows. If it does not, connect a DC ammeter in series with the fan to see how much current it draws if it will run momentarily before blowing the fuse. If the current being drawn is higher than the fuse rating, it is likely the fan causing the problem.

If the fuse blows with the fan disconnected, then there is indeed a short circuit somewhere. You will probably find it by visually examining all portions of the wiring route.