You may find a bag with a fuse and a relay in your truck when you take delivery. Mine was in the drivers door map pocket, but it may be in the glovebox. The bag was marked with the part number AL3T-15A416-AA. These provide power to the trailer battery charge pin located on the 7-pin trailer plug. Install the relay into position 9 of the underhood fuse box located on top of the radiator. Install the fuse into position 21. The circuit has a capacity of 30A.
If you have a backup camera, you may find another plug in the glove box. This plug is used to cap the camera wiring connector in case you remove the tailgate.
This is for a 2011, FWIW. On my 2001 there were several fuses and relays included in the bag with the brake controller pigtail. These fuses and relays powered the trailer battery charge circuit, the electric trailer brake battery feed circuit and the reverse light circuit. These went in the underhood fuse box located on the drivers side fenderwell.
2011 SVT Raptor SuperCrew
2008 G37S 6MT Black/Black
2001 SuperCrew Lariat 4x4 (Sold 3/11)
Deep Wedgewood Blue/Graphite Captains chairs
5.4L, trailer tow, sunroof, bed extender and 1.42 radiator.
Mods: Expedition console Edge EVO and Outlaw CAI.