Have a 2006 F250 with a crew cab.
I am going to put a 12v power point in the back seat.
I thought I had read a past post that said the harness might already be in the truck.

1. Where is a good spot to start looking for it?
2. On the factory install, where was it installed?

 

Hmmm.. get a hold of a friends truck and see.

If the factory outlet is used it is probably controlled by the BCM.

I know the E series power plug beside the driver (left side) is.

 

If a 2005 vehicle would be close enough to yours, go to autozone's site, register, and then plug in a 2005 f250 crew cab. Then click on "vehicle repair guides" and then click on "wiring diagrams". Scroll down the right side, and they have "component views" and "connector views". These may give you hints on where to look.