I just bought a 2010 Screw Lariat last week and when I took delivery of it the salesman was showing me some things about the nav, radio, etc. When I looked in the glove box there was a small plastic bag that had a relay and a fuse in it. I asked him what it was for and he said "probably just a spare". I bought a camper top for the '10 yesterday and this morning was out wiring up the third brake light and the dome light in it. On my '05 I ran a length of 12/2 extension cord through the frame, connected it to the battery with a 30 amp inline fuse and at the rear, inside the bed, I mounted a double power outlet to plug in an inverter, etc. I then connected the dome light for that camper to that same power source. I got to thinking this morning that the 7-pin plug for the trailer connection should have a 12v+ terminal for charging accesories when pulling a trailer. I checked that pin and it had no power. Went and looked in the manual and sure enough, under the hood in the power dist. bos, there is a spot for the relay and the fuse (#9 is the relay and #21 is the fuse if I remember right). I now have a 12v+ connection that I can hook the dome light to at the back of the 7-pin plug without running the wire from the battery. The power supply is switched which menas the key has to be on but thats not a real biggy.
I now have one 4way flat trailer connection that stays in the truck and the other end on the camper that I can plug up when the camper is on and I have a neat hookup for the lights. (2 for the third brake light and 2 for the dome light/power supply).
Thought this might help someone who was installing the same or just wanted to know what the spare parts were really for.

 

Well, congratulations on the new truck and thanks for letting us know what those are for. I've heard where they get plugged in before, but not what they do.

 

hey thanks for that. I was thinking of how to wired a trailer that I'm customizing so that I can put a inverter and other power outlets for accessories and computers and also keep a spare battery and being able to charge it when the truck was running. thanks


by the way my salesman told me that was for somethin like electric trailer brakes or something.

 

You're welcome. The reason it's switched is to provide a charging port for the breakaway batteries on trailers that have them (If the trailer comes unhooked while in tow, the breakaway switch locks the electric brakes on the trailer). When I used to drag race a lot, I had mine unswitched so it powered the 12v lights in the trailer as well as the stereo and a 12v water pump for my sink.

Happy New Year!

 

the fuses are for the 7 pin connector under the rear bumper, to make the trailer hook up work properly (charging , electric brakes)

as for the wiring it plugs in to a port under the dash and is for the purpose of a brake controller (aftermarket) if you have a 09 + f150 with both 4 pin flat and 7 pin round plugs and a factory receiver under rear bumper you can add a factory brake controller to the dash!

OP as for using this plug for a camper power/brake lights sound like a neat idea.
you do know thay make a whip that plugs in and dose all that you have said you did for like $30 bucks and can plug right in to most campers! that is if its a camper and not just a shell