I have two of these. One in front and one in back. The one in rear is to cool the 50% of your body that's not affected by the air coming from the front. I use it on longer/harder rides or races. Has a noticeable effect on sustained power output.
Second thing is that I have this keyfob remote plug so I can turn either fan off or on without getting off bike or even slowing down. It has three switches so it controls its three plugs individually. Wonderful! Amazon remote control plug
Something like this wireless switch should do the trick.