You already know the fan runs with DC applied to the fan.
If you can use a volt meter, test for voltage at the fan using AC (alternating current) and DC (direct current) scales. It should be DC. Of course, the bike should be at temp so the thermo switch is on.
If you are not familiar with a volt meter, you could also simply connect a 12 volt light bulb in place of the fan. If it lights, you have power. The bulb will light with either AC or DC.
If DC, the fan should work. If AC, the fan will not work. The rectifier is what converts AC to DC power. So, if you find AC power, replace the rectifier and you should be back riding.