The problem isn't turning the fan on, or turning it off.

The fan mod is done by grounding the same wire as the pcm to the high speed fan relay on the car. Any switch will do and work fine. Some of us forgetful people see our temps lowered and don't turn the switch off. So far no problem, car is turned off and fans turn off. Upon the next restart the fans turn on and the pcm diagnostic sees the fan on. The service engine soon light comes on to tell you there's a DTC code to help you troubleshoot not turning the switch off to your fan override.

Here's the part, yes a single relay latching itself from momentary switch would work. I'm into options, I want to be able to turn it off and on from a single switch. And a single relay can not do that.

I wanted a latching on/off system and chose to pick up some relays at the junkyard. (Cost: Free) Hook then together properly according to the latching/unlatching setup. I hook the power to ignition power and it automatically resets when the car is turned off.

Diagrams can be found here. 4th or 5th will do a postive output. Add one relay to output ground.
http://www.the12volt.com/relays/page5.asp
Or it could be done via circuitry using an IC flip flop chip
http://www.imagineeringezine.com/PDF-FILES/4013oneshots.PDF
I was unable to appropriate a flip flop yet and therefore haven't finished that part of my project.

Contrary to what some may think... I have not lost touch with reality, rather; reality cannot keep up with me