[Rev]

I just ran mine to the starter relay. It has a terminal thats on when the ignition is on. It's right there on your passenger side fender apron on the '66. Now these fans draw a fair number of amps (10-15 ?), so I recommend using a 30 amp relay to activate the fan. That way, you don't have to run all that juice through the ignition switch. You can get that 30 amp relay for under $10 at PepBoys or Radio shack. I've had better luck with the ones from PepBoys. The relays come with wiring instructions.

Rev