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The above advice is all valid. Personally, I would install a second relay that got its main power from fuse 3 and its pull-coil power from the hot lead (the wire that goes to the main fan) of the first relay. Run the other side of the relay pull-coil connection through the new switch to ground. You'll end up with front fans that only run when the rear fan, the ignition, and the new switch is on.
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