From the Ford Truck Enthusiast Forum:
"Fuse #20 in the power distribution box (located under the hood) is a 10A fuse for the trailer back-up lights (reference page 223 of the 2005 owner's manual).
The fuse which powers the trailer brake controller is #20 in the passenger compartment panel and is a 15A fuse (reference page 219 of the 2005 owner's manual)."
Hope this helps!!!
〉 Answered on Sep 21st, 2012 by Karen Salvaggio, Owner - Professional Race Car Driver at Thunder Valley Racing
Alicia, depending on the month of manufacture and the engine size, this could vary. But what I did find was this: #20(top) - 10amp Trailer Tow Stop and Left Turn Lamp. Otherwise, if you look in your owners manual, that would be the most accurate way to verify.