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Parallel Fan Wiring
When the AC comes on the second fan engages.  If you run the fans in parallel they both come on via the engine's temperature.  This provides more cooling....esp. for those of us that are forced induction or live in hot areas.  Don't worry you don't need the second fan to kick in with the AC...engine temperature control is quite sufficient.  Also note that you should NOT hook up the old AC triggered hot wire...it goes to ground when the AC is off. 

 
This is the AC fan's plug. I cut in here to bring the hot wire over from the motor fan.
 
Here's the hot wire on the motor fan side.
 
I slit it with a single edge razor blade...BE CAREFUL!
 
The tap is made...soldered of course. You only need the hot wire.
 
I put a connector at the other end, just where I hooked up to the AC side. This is necessary for removing the fans to work on the front of the engine.
 
Crimping is great. It holds the wire so you can solder it! ;-)
 
There, ALL DONE! NOTE: Do NOT re-hook up the original secondary fan's hot wire.  It goes to ground when the AC is off and will short out your fuse.
 
Test the fans by jumping GND and TFA in the diagnostic box. They should BOTH come on.
 
BTW here is a great place to store your paper clip. You know you've run into a real miataphile when they see it under the hood and go, "Hey that's a great place to store that jumper!"  ;-)