You will need to solder 1/4 watt resistors to the control relays. These resistors apply the proper load to the PCM output so that no SES code is set when the fans are manually controlled.
The LIGHT BLUE wire in the drawing is the only wire that needs to be passed through the firewall. Use a minimum of 18AWG wire for this modification, and use proper inline fuses on the + voltage lines to prevent damage in the event of a short circuit.
The relays are generic 1" cube automotive 30A relays that have an internal coil resistance of approximately 96 Ohms.
You will leave the stock relays in the relay center. There is actually no need to even open up the cover. This modification will require 3 additional relays shown below that can be purchased at almost any automotive parts store. They are most commonly used to control auxiliary wiring.
You also may be wondering why I didn't design the circuit for HIGH/LOW operation. This would require an additional switch, or a more complicated one. Secondly, are you trying to cool down or what? In my opinion, there is no point for having a "low" setting. You are either going to cool the radiator down or you are not. Simple as that.
