With not even a hint what the hardware is, it will be tough to be much help, but I will tell you that I have never seen an individual fuse for a single signal light.
My personal suspicion would be that when the trailer harness was spliced in, a poor job was done and it has now failed. This could be a simple fix if you can find that splice. With most vehicles, the taillight harness comes down the left side of the vehicle and there is a place in the left rear corner where there is the split off to the right rear light and that is usually where I attacked the harness to put in a trailer harness.
Matt