Tail Light Fuse Keeps Blowing

[upd103]

OK. About a month ago, the tail light fuse on the truck blew when my wife pulled the horse trailer.

She replaced the fuse, and pulled the trailer back without any problems and without blowing a fuse. Everything was good.

She pulled the trailer again yesterday, and once again, blew the fuse. This time though, the new fuses blow as soon as she replaces them. Literally as soon as it makes contact.

What do y'all think is wrong? Do you think one of the relays under the hood took a crap and is causing the short? I don't think it's the trailer because it pulled fine on the way back from Dallas last month. And now, the fuse is blowing without the trailer even being connected.

The relay is the only thing I can think of. What do y'all think?

BTW, I'm in Afghanistan, and the wife is in TX, so that's why I haven't personally gotten to play with it.

David