Which fuse or relay is most likely the cause for my trailer lights not working? I replaced F42 which was burned out and F36 works, but I can't get the trailer lights or brake lights to activate.

I thought perhaps it was relay 201 which is on the left fender but I don't know how to open the black box.

 

Which model 2004 do you have, a Heritage ( same body style as the 99-03 ) or a new body style ?

I am guessing a 2004 Heritage, with the reference to the Battery junction box on the left fender, but then the fuse numbers are from a new body style truck.

 

on my 1998 the fuse was on the left fender like SSCULLY says. It doesn't even look like a fuse box so it is hard to find. Mine was also taped shut.

 

It is the new style body. It was purchased in May of 2004.

 

How can a person determine if the relay isn't working? All I know is that the trailer lights (clearance, brake, and stop) do not work. Any help will be appreciated. The turn signals work.

 

The S/T/T on the trailer are not working. I read "trailer lights" and made a bad assumption that this meant the parking lamps.

I would start with a meter and a different ground in the back of the truck, and check for voltage at the connector ( maybe check the connector first to make sure it is clean, and not corroded ).
The relays for the trailer tow are in the passenger cab now, there is no battery junction box under the hood on the '04 + MY trucks ( a small aux fuse panel, and relay 201 in that is for the DRLs ).

Start with checking the connector against a different ground, it could be the ground is bad, or the plug end is corroded beyond use.

If you do not have your owners manual you can download one :
https://www.fleet.ford.com/maintenan...ls/default.asp

In the Towing and Hauling forum, there is a sticky at the top with the relay and fuse locations from the Trailer tow install directions.
https://www.f150online.com/forums/to...locations.html