Just for Info;
On a 2010 Ford F150 the relay PN is 8T2Z 14N089 A it goes in Spot number 9 in the under hood relay\fuse box, the fuse can be obtained at an Auto Parts store and some other General Stores, it is 30 AMP, it goes in Position 21.
There is a short cable that can be replaced to make the truck 7 AND 4 pin. The Ford PN is 9L3Z 13A576 BB. This cable replaces the stock one that is 4 pin only. The socket for this cable is a Hopkins brand 7 and 4 PIN that you can get at Northern Tool, and some Walmarts.
The cable is plugged into the rear wiring harness, above the rear Axle. While your at it move the Frame Ground to a more accessible place. It is easiest to remove the spare tire while doing this.
I installed a Curt Class 4 hitch and this wiring arrangement. I now have 7 and 4 pin and trailer battery charging on the 7 Pin. I also found the brake control plug in above the brake pedal under the dash, it is taped to a bundle of wires there, I have yet to need or connect that.
These Ford Parts allow you to do this modification with NO cutting or splicing of wires. A little dielectric grease and away you go.
Please be careful with electrical, and only do what you are comfortable with. If in doubt leave it to a professional.