It depends if you want a switched feed or not.

off the coil would give you power only if your ignition is on.
the back of the alt would be constant power... similarly the starter motor usually offers a source for constant power.

the best would be to get power thru a relay.

Generally fans draw a fair amount of load.

I would set up a relay which could be set up (wtih fuse) back to a constant power source. this ensures the load requirements can be met.

the switch can be then set up on the low power side of the relay.

your alt suggestion is fine.. just make sure it's fused.

Hope this helps.

Cheers

Ed