You can still have them come on with the ignition switch using a relay (wiring 101), The idea is to pull your power from the source not the alternator, using a relay is eliminating running high voltage AMPERAGE not voltage past the firewall and having the shortest power source to the fans, Maybe a read on basic automotive wiring is in order.
It is high amperage through a switch we are trying to avoid. The Dakota Digital controller probably has a built-in mechanical relay, or mosfet switch.
Do you have that exciter wire hooked up? That is the first step here.
