Now I've done some searching and I've even looked at Jime's website, and I can't come to a conclusion on exactly how to go about doing this. I'm wiring up a switch for use in the cabin, primarily for drag racing obviously. I am confused, however. Jime outlines how to do it two COMPLETELY different ways on here and on his website, using two different methods, two different switches, etc.

I'd like to go with the way he does it on his website, but he is a little vague. I see which pins control the fans, and understand how I'm to use the switch, but, are there wires coming out of the ECU at these two pins? If there are, and I am to use them, how does an SPDT switch allow for an "auto" mode? What I mean is, the SPDT switch seems to only allow for manual control (either low or high speed) and doesn't seem like it allows the ECU any control over the fans once you're done manually controlling them.

Here is the page for reference if anyone is unfamiliar with it:
http://jime.homeip.net/fan.html

How does this thing get wired up?

 

I think the part you are missing is that you are not cutting the wires to the ecu, you are just tapping into them.

You just provide a manual ground with the switch and when the contact(s) are open, the fans operate normally.

 

I see, I see. Makes sense now. T-ing in rather than interrupting. Thank you, sir!