I am not a mechanic by any means but I believe..

The stock fans have a two stage circuit with what I believe is a motor inside a motor. One turns on at 180 degrees and the other at 200 I believe. If you hook up to one circuit you get a fan that works from 180-200 and then shuts off. If you hook up to the other it doesn't work until 200. It is best to run a new circuit from a constant 12v with a temp sensor located near the top hose of the radiator because the bottom hose is too cold.

I didn't figure this out on my own. I had the same issue and wickedesi explained the diagram out of the service manual to me.