I have a new horse trailer which has switched interior lights. The interior ligths don't work but all of my turn signals and brake lights work well. The trailer dealer says that I need the auxiliary power set up in the truck. My 04 has the factory towing option. I read other posts on the subject and checked my fuses which seem to be good. The only relay I found missing was the r05 relay for trailer battery charge.

What fuse will provide me auxiliary power for my trailer?

BTW, what Ford genius put the fuse box in a place where its really hard to see (and work on)?