for those that say, "why would you want to have a manual switch - let the controller do it" - you are lucky. if you have had a controller/fan switch fail, you would think differently. i use them if im sitting in horrible traffic, waitin for a train etc. i would prefer to start cooling sooner rather than having the thing get pretty hot and having to play 'catch up'.
Bernie, as usual, is right on. tap into the trigger from your controller to the relay. a blocking diode might be a good idea too, but probably not necessary. very simple to do.
good luck.