This is on a 2005 F-150 Lariat so it could be different for different model years. I looked on the forums for 2 days to nail down the location of this wire. I saw different posts some saying the wire was green, others saying it was black and pink. All posts agreed it was in the kick panel but they could not agree if it was on the passenger side or driver side.
I finally found my wire which when your truck is reverse and the key is on, becomes hot and will activate a reverse/backup camera and or the backup feature on your double din head units.
Mine was located on the drivers side kick panel. There is a grey connector secured with a bolt through the center. If you look towards the back of the connector (towards the firewall) you will fine a black wire with a pink... almost purple stripe. I probed the wire and determined it only becomes hot when the truck is in reverse and the ignition is on. I tapped into the wire, and then taped it off.
I took a few photos in hopes that I could spare someone else the trouble of probing a ton of wires "I must of probed 10 different wires combined". Hope this helps someone.
2005 Ford F-150 Screw
Cervinis RamAir Hood, Harley Headlights, Aftermarket Rear and Third brake light, 3" Lift, Gibson Exhaust, K&N
Drop in, Painted Factory 18's, Interco M16 Super Swampers, LineX,