I'm sure this has been asked a lot, I am hooking up an electric fan from scratch. I got the relay kit, fan, and switch. My question is what would be a good location to hook up the switched 12v wire? Is there somewhere I could tap off of in the engine bay? Where are all the switched 12v spots in the engine bay? thank you in advanced.

The pink w /blk stripe wire circuit.

Where is that wire located? What is it coming off of, or powering? Is it a 12v switched wire?

It's everywhere. All over under the hood. Just look around at connectors. There's one at the original electric fan relay, a couple at the smog pump, probably 5 or 6 more at least.

I installed a motor from a c-10 pickup, I did not use the knock sensor, can I use that wire for the switched on? Sorry for not following, but I don't have a ton of experience with wiring vehicles.

No the knock sensor wire won't work.

There are all kinds of things all around the engine that require power. They all have a wire that's on that circuit. Just look at connectors, one by one. Solenoids and relays are particularly good places to find that circuit. It's kind of like, if you wanted to add a light inside your car somewhere that would come on when you opened the door, you wouldn't ask "which wire"; there's like A DOZEN wires you could find that any of which would do what you need because they're all hooked together and do the same thing, so all you need to know is WHAT COLOR WIRE since all the wires that do the same thing are the same color. Same deal here. You need a wire on the pink w/ black stripe circuit.

Find a pink w/ black stripe wire in some connector somewhere; follow it back to some convenient spot INSIDE the split-loom tubing if possible; splice onto it by stripping the insulation WITHOUT CUTTING IT, wrap the wire for your new relay around it, solder it, tape it back up REAL GOOD; put it back into the split-loom.

Doesn't matter what the cast iron in the motor came out of. Your car's wiring has what you need. Assuming of course, that nobody "improved" the car too much by "cleaning up the engine bay" (hacking on stuff they didn't know what it was for) whenever the motor was replaced.