what is the power source for an electric choke?

It should go to a temperature sensor, most likely on the top end of the motor. As the coolant warms up and the sensor "closes" per say it allows voltage to the bimetallic spring in the choke module. This is not specific to Ford but common knowledge ive gained over the years. I apologize if its not accurate to your application

I would think it is wired to come on with the ignition. So I would expect it to be wired on the primary side of the coil. Primary is the 12 volt side. Connect it to whichever terminal has 12 volts when the ignition is turned on. The other terminal would go to the breaker points in the distributer.

1966 Ford did not have an electric choke. That being said, its an easy fix. find the fuse box there should be a slot marked ing (ignition) run a wire from there to the choke.

I just realized I was explaining how a vacuum actuated choke works minus the bimetallic spring, my apologies.

@Brianator Your answer does not make sense, If you wire it to the temp sensor the overheat light will come on. @ Hornet Really? Hook it to the 12 volt side of the coil? That should produce some interesting results Coils have Ballast resistors wired in series on the hot side Not 12 volts

And that's exactly why I made the correction. There are coolant vacuum port sensors for vacuum actuated choke assemblies, I mixed the 2 together for some reason. Lol