I don't think there is any switched source under the dash, other than the the power to the start (S) side of the solenoid during starting. Trouble is, unless you use a relay, it could feed back and try to operate the starter all the time. Just go with the I terminal. The solenoid IS a relay.

 

Thanks! "I" side it is.

 

Is the "I" terminal not a switched 12v source when the key is in the ON position and not running?

 

I think it's only 12v while cranking. Otherwise, you get whatever comes through the resistor wire.

 

Shouldn't be. Test it.

 

I'd say the I is a good place to wire it. Just like wiring it straight behind the ignition switch.

 

I ran a relay when I installed my HEI. I am no expert, but I could not find a good 12 volt source. With the relay it works perfect.

 

When I turned the key to the ON position to run through my efi handheld, it didn't turn on. I found a different source in my AAW kit.

 

The_Dude is correct.
The I terminal will get back-feed from the coil (pink wire) when the engine is running - unless you install a diode to stop it. It's purpose is to supply 12V to the coil during starting.
The S terminal should only have 12V when the key is in the start position as it is the wire that 'fires' the solenoid.

 

If you can deal with the current being back-fed from the coil, fine, but I'd install a fuse-protected relay switched by the "S" terminal. You know it'll be clean power and off when the key is released. Diodes can, and do, fail.

 

My MSD box is on the back of the ignition switch if I remember correctly. Or on the heater, it's been 12 years. I know the back of the switch on my Galaxie cuts off during start.