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as the title ask's ..
15' SD Lariat
I was hoping for a simple source, like in the 4 pin trailer wiring ?
If i can do it that way, can you tell me which wire it is ?
I would never use scotch locks to butcher a factory harness. And I wouldn't butcher the light harness unless it was an emergency roadside repair.
My '99 has a clearly marked green CHMSL wire hanging out of the light harness next to the rear bumper. Not sure when or if they stopped running it all the way back on the the Super Duties, but I know on my 1997-03 trucks they all have had it back there but at some point on the F150s the green wire stopped either under the cab near the front of the bed or at the firewall somewhere. I guess the plan was for the installer to run that wire along the camper instead of under the bed, I don't know. But I do know at some point they stopped running it all the way back but it is still there somewhere.
You should be able to trace that harness back and find it. If it isn't run all the way back then most likely it will either be right at the master cylinder or ABS module in the engine compartment or under the cab.
Edit: "If only stop lamp function is desired for the added lights, connect to circuit YE-GN CLS43 at the blunt cut customer access wire located at the rear of the vehicle near trailer tow connector C4099."
Found in this document: https://www.fleet.ford.com/truckbbas...iring_v1-0.pdf
So this will allegedly be a yellow wire with a green stripe at the back of the vehicle either taped to or near the existing factory trailer tow harness instead of the green wire on my '99 as shown in my picture.
I personally don't use scotch-loks. The video was a easy way to show where he can tap in for his brake light.
I still think it is easier to use the wire that supplies on every truck for this exact purpose instead of tapping into the factory harness. I don't want to seem disagreeable or argumentative about it, but is my opinion that would be the simplest and best way to do it. And I base that on my experience of
A) owning used trucks that I have had to redo the harness where a previous owner has tapped into the harness unnecessarily. I would prefer not to repeat that experience of night time roadside repair just to get tail and brake light functionality back, and I don't recommend it to anyone else.
and
B) installing and wiring my own camper shell with the wire supplied for this purpose and never having any problems with it whatsoever.
I added a small 12V battery out of a computer backup power supply and isolated it with a diode so that the brake wire charged the battery whenever the brake light was lit. I then tapped off this additional battery to power the camper shell dome light. It worked great, it would run the dome light for days if someone forgot to turn it off and that light was the only thing affected. The brake lights all still worked.
Believe it or not, I do agree with that. As a matter of note, I run the S/T/T/R lights on my headache rack from a Curt T-one connector on my trailer harness. No splices in the factory harness.
Thanks so very much for the information ..I truly appreciate your time .!!
It seems to answer all the questions I've had about adding additional power sources ..
supplies on every truck for this exact purpose instead of tapping into the factory harness. I don't want to seem disagreeable or argumentative about it, but is my opinion that would be the simplest and best way to do it. And I base that on my experience of 
