Electric cooking fan...

Sorry, I should have been more clear... I ordered the derale switch, the one that comes with the temp sender you push through the fins of the rad.

I'm just not sure where I should pick up an ignition wire from under the hood...

I have no idea what you're doing...........................?

But there is only one switched ignition under the hood, and that is........"switched ignition." Which comes out of the bulkhead, is traditionally dark blue, and feeds the igntion system and regulator IGN terminal on 69/ earlier cars, and also feeds alternator field on later cars

Depending on "what you want" it for, use the IGN wire to trigger a relay to run your accessory, and fuse it off the starter relay battery stud.